first commit

core/modules/language/tests/language_config_override_test/language_config_override_test.info.yml

@@ -0,0 +1,10 @@
+name: 'Language config override test'
+type: module
+description: 'Support module for the language config override test.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/language/tests/language_elements_test/language_elements_test.info.yml

@@ -0,0 +1,12 @@
+name: 'Language form elements test'
+type: module
+description: 'Support module for the language form elements tests.'
+package: Testing
+# version: VERSION
+dependencies:
+  - drupal:entity_test
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/language/tests/language_elements_test/language_elements_test.routing.yml

@@ -0,0 +1,15 @@
+language_elements_test.config_element:
+  path: '/language-tests/language_configuration_element'
+  defaults:
+    _form: '\Drupal\language_elements_test\Form\LanguageConfigurationElement'
+    _title: 'Language configuration form element'
+  requirements:
+    _access: 'TRUE'
+
+language_elements_test.config_element_test:
+  path: '/language-tests/language_configuration_element_test'
+  defaults:
+    _form: '\Drupal\language_elements_test\Form\LanguageConfigurationElementTest'
+    _title: 'Language configuration form element'
+  requirements:
+    _access: 'TRUE'

core/modules/language/tests/language_elements_test/src/Form/LanguageConfigurationElement.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Drupal\language_elements_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\language\Entity\ContentLanguageSettings;
+
+/**
+ * A form containing a language configuration element.
+ *
+ * @internal
+ */
+class LanguageConfigurationElement extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_elements_configuration_element';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $conf = ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'some_bundle');
+
+    $form['lang_configuration'] = [
+      '#type' => 'language_configuration',
+      '#entity_information' => [
+        'entity_type' => 'entity_test',
+        'bundle' => 'some_bundle',
+      ],
+      '#default_value' => $conf,
+    ];
+
+    $form['submit'] = [
+      '#type' => 'submit',
+      '#value' => 'Save',
+    ];
+    $form['#submit'][] = 'language_configuration_element_submit';
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}

core/modules/language/tests/language_elements_test/src/Form/LanguageConfigurationElementTest.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\language_elements_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * A form containing a language select element.
+ *
+ * @internal
+ */
+class LanguageConfigurationElementTest extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'language_elements_configuration_element_test';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['langcode'] = [
+      '#title' => t('Language select'),
+      '#type' => 'language_select',
+      '#default_value' => language_get_default_langcode('entity_test', 'some_bundle'),
+    ];
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}

core/modules/language/tests/language_entity_field_access_test/language_entity_field_access_test.info.yml

@@ -0,0 +1,16 @@
+name: 'Language entity field access test'
+type: module
+description: 'Support module for verifying entity field access and the language selector.'
+package: Testing
+# version: VERSION
+dependencies:
+  - drupal:node
+  - drupal:text
+  - drupal:field
+  - drupal:filter
+  - drupal:language
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/language/tests/language_entity_field_access_test/language_entity_field_access_test.module

@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * @file
+ * Hook implementations for language_entity_field_access_test.
+ */
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Implements hook_entity_field_access().
+ */
+function language_entity_field_access_test_entity_field_access($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, ?FieldItemListInterface $items = NULL) {
+  return AccessResult::forbidden();
+}

core/modules/language/tests/language_events_test/language_events_test.info.yml

@@ -0,0 +1,9 @@
+name: 'Language events test'
+type: module
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/language/tests/language_events_test/language_events_test.services.yml

@@ -0,0 +1,6 @@
+services:
+  language_events_test.event_subscriber:
+    class: Drupal\language_events_test\EventSubscriber
+    arguments: ['@state']
+    tags:
+      - { name: event_subscriber }

core/modules/language/tests/language_events_test/src/EventSubscriber.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace Drupal\language_events_test;
+
+use Drupal\Core\State\StateInterface;
+use Drupal\language\Config\LanguageConfigOverrideEvents;
+use Drupal\language\Config\LanguageConfigOverrideCrudEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+class EventSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Constructs the Event Subscriber object.
+   *
+   * @param \Drupal\Core\State\StateInterface $state
+   *   The state key value store.
+   */
+  public function __construct(private StateInterface $state) {
+  }
+
+  /**
+   * Reacts to config event.
+   *
+   * @param \Drupal\language\Config\LanguageConfigOverrideCrudEvent $event
+   *   The language configuration event.
+   * @param string $event_name
+   *   The event name.
+   */
+  public function configEventRecorder(LanguageConfigOverrideCrudEvent $event, string $event_name): void {
+    $override = $event->getLanguageConfigOverride();
+    $event_info = [
+      'event_name' => $event_name,
+      'current_override_data' => $override->get(),
+      'original_override_data' => $override->getOriginal(),
+    ];
+
+    // Record all events that occur.
+    $all_events = $this->state->get('language_events_test.all_events', []);
+    $override_name = $override->getName();
+    if (!isset($all_events[$event_name][$override_name])) {
+      $all_events[$event_name][$override_name] = [];
+    }
+    $all_events[$event_name][$override_name][] = $event_info;
+    $this->state->set('language_events_test.all_events', $all_events);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    $events[LanguageConfigOverrideEvents::SAVE_OVERRIDE][] = ['configEventRecorder'];
+    $events[LanguageConfigOverrideEvents::DELETE_OVERRIDE][] = ['configEventRecorder'];
+    return $events;
+  }
+
+}

core/modules/language/tests/language_test/language_test.info.yml

@@ -0,0 +1,10 @@
+name: 'Language test'
+type: module
+description: 'Support module for the language layer tests.'
+package: Testing
+# version: VERSION
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/language/tests/language_test/language_test.module

@@ -0,0 +1,110 @@
+<?php
+
+/**
+ * @file
+ * Mock module for language layer tests.
+ */
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUI;
+
+/**
+ * Implements hook_page_top().
+ */
+function language_test_page_top() {
+  if (\Drupal::moduleHandler()->moduleExists('language')) {
+    language_test_store_language_negotiation();
+    \Drupal::messenger()->addStatus(t('Language negotiation method: @name', ['@name' => \Drupal::languageManager()->getNegotiatedLanguageMethod() ?? 'Not defined']));
+  }
+}
+
+/**
+ * Implements hook_language_types_info().
+ */
+function language_test_language_types_info() {
+  if (\Drupal::state()->get('language_test.language_types')) {
+    return [
+      'test_language_type' => [
+        'name' => t('Test'),
+        'description' => t('A test language type.'),
+      ],
+      'fixed_test_language_type' => [
+        'fixed' => ['test_language_negotiation_method'],
+        'locked' => TRUE,
+      ],
+    ];
+  }
+}
+
+/**
+ * Implements hook_language_types_info_alter().
+ */
+function language_test_language_types_info_alter(array &$language_types) {
+  if (\Drupal::state()->get('language_test.content_language_type')) {
+    $language_types[LanguageInterface::TYPE_CONTENT]['locked'] = FALSE;
+    unset($language_types[LanguageInterface::TYPE_CONTENT]['fixed']);
+    // By default languages are not configurable. Make
+    // LanguageInterface::TYPE_CONTENT configurable.
+    $config = \Drupal::configFactory()->getEditable('language.types');
+    $configurable = $config->get('configurable');
+    if (!in_array(LanguageInterface::TYPE_CONTENT, $configurable)) {
+      $configurable[] = LanguageInterface::TYPE_CONTENT;
+      $config->set('configurable', $configurable)->save();
+    }
+  }
+}
+
+/**
+ * Implements hook_language_negotiation_info_alter().
+ */
+function language_test_language_negotiation_info_alter(array &$negotiation_info) {
+  if (\Drupal::state()->get('language_test.language_negotiation_info_alter')) {
+    unset($negotiation_info[LanguageNegotiationUI::METHOD_ID]);
+  }
+}
+
+/**
+ * Store the last negotiated languages.
+ */
+function language_test_store_language_negotiation() {
+  $last = [];
+  foreach (\Drupal::languageManager()->getDefinedLanguageTypes() as $type) {
+    $last[$type] = \Drupal::languageManager()->getCurrentLanguage($type)->getId();
+  }
+  \Drupal::keyValue('language_test')->set('language_negotiation_last', $last);
+}
+
+/**
+ * Implements hook_language_fallback_candidates_alter().
+ */
+function language_test_language_fallback_candidates_alter(array &$candidates, array $context) {
+  if (Drupal::state()->get('language_test.fallback_alter.candidates')) {
+    unset($candidates[LanguageInterface::LANGCODE_NOT_SPECIFIED]);
+  }
+}
+
+/**
+ * Implements hook_language_fallback_candidates_OPERATION_alter().
+ */
+function language_test_language_fallback_candidates_test_alter(array &$candidates, array $context) {
+  if (Drupal::state()->get('language_test.fallback_operation_alter.candidates')) {
+    $langcode = LanguageInterface::LANGCODE_NOT_APPLICABLE;
+    $candidates[$langcode] = $langcode;
+  }
+}
+
+/**
+ * Implements hook_module_preinstall().
+ */
+function language_test_module_preinstall() {
+  \Drupal::state()->set('language_test.language_count_preinstall', count(\Drupal::languageManager()->getLanguages()));
+}
+
+/**
+ * Implements hook_language_switch_links_alter().
+ */
+function language_test_language_switch_links_alter(array &$links, $type, Url $url) {
+  // Record which languages had links passed in.
+  \Drupal::state()->set('language_test.language_switch_link_ids', array_keys($links));
+}

core/modules/language/tests/language_test/language_test.routing.yml

@@ -0,0 +1,33 @@
+language_test.l_active_class:
+  path: '/language_test/type-link-active-class'
+  defaults:
+    _controller: '\Drupal\language_test\Controller\LanguageTestController::typeLinkActiveClass'
+  requirements:
+    _access: 'TRUE'
+
+language_test.subrequest:
+  path: '/language_test/subrequest'
+  defaults:
+    _controller: '\Drupal\language_test\Controller\LanguageTestController::testSubRequest'
+  requirements:
+    _access: 'TRUE'
+
+language_test.entity_using_original_language:
+  path: '/admin/language_test/entity_using_original_language/{configurable_language}'
+  defaults:
+    _controller: '\Drupal\language_test\Controller\LanguageTestController::testEntity'
+  requirements:
+    _access: 'TRUE'
+
+language_test.entity_using_current_language:
+  path: '/admin/language_test/entity_using_current_language/{configurable_language}'
+  defaults:
+    _controller: '\Drupal\language_test\Controller\LanguageTestController::testEntity'
+  requirements:
+    _access: 'TRUE'
+  options:
+    parameters:
+      configurable_language:
+        type: entity:configurable_language
+        # Force load in the interface text language selected for page.
+        with_config_overrides: TRUE

core/modules/language/tests/language_test/src/Controller/LanguageTestController.php

@@ -0,0 +1,139 @@
+<?php
+
+namespace Drupal\language_test\Controller;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Language\LanguageManagerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Drupal\language\ConfigurableLanguageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+
+/**
+ * Controller routines for language_test routes.
+ */
+class LanguageTestController implements ContainerInjectionInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The HTTP kernel service.
+   *
+   * @var \Symfony\Component\HttpKernel\HttpKernelInterface
+   */
+  protected $httpKernel;
+
+  /**
+   * The language manager service.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Constructs a new LanguageTestController object.
+   *
+   * @param \Symfony\Component\HttpKernel\HttpKernelInterface $httpKernel
+   *   An HTTP kernel.
+   * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
+   *   The language manager.
+   */
+  public function __construct(HttpKernelInterface $httpKernel, LanguageManagerInterface $language_manager) {
+    $this->httpKernel = $httpKernel;
+    $this->languageManager = $language_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static($container->get('http_kernel'), $container->get('language_manager'));
+  }
+
+  /**
+   * Route entity upcasting test helper.
+   *
+   * @param \Drupal\language\ConfigurableLanguageInterface $configurable_language
+   *   The ConfigurableLanguage object from the route.
+   *
+   * @return string
+   *   Testing feedback based on (translated) entity title.
+   */
+  public function testEntity(ConfigurableLanguageInterface $configurable_language) {
+    return ['#markup' => $this->t('Loaded %label.', ['%label' => $configurable_language->label()])];
+  }
+
+  /**
+   * Returns links to the current page with different langcodes.
+   *
+   * Using #type 'link' causes these links to be rendered with the link
+   * generator.
+   */
+  public function typeLinkActiveClass() {
+    // We assume that 'en' and 'fr' have been configured.
+    $languages = $this->languageManager->getLanguages();
+    return [
+      'no_language' => [
+        '#type' => 'link',
+        '#title' => t('Link to the current path with no langcode provided.'),
+        '#url' => Url::fromRoute('<current>'),
+        '#options' => [
+          'attributes' => [
+            'id' => 'no_lang_link',
+          ],
+          'set_active_class' => TRUE,
+        ],
+      ],
+      'fr' => [
+        '#type' => 'link',
+        '#title' => t('Link to a French version of the current path.'),
+        '#url' => Url::fromRoute('<current>'),
+        '#options' => [
+          'language' => $languages['fr'],
+          'attributes' => [
+            'id' => 'fr_link',
+          ],
+          'set_active_class' => TRUE,
+        ],
+      ],
+      'en' => [
+        '#type' => 'link',
+        '#title' => t('Link to an English version of the current path.'),
+        '#url' => Url::fromRoute('<current>'),
+        '#options' => [
+          'language' => $languages['en'],
+          'attributes' => [
+            'id' => 'en_link',
+          ],
+          'set_active_class' => TRUE,
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Uses a sub request to retrieve the 'user' page.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The kernels response to the sub request.
+   */
+  public function testSubRequest() {
+    $request = Request::createFromGlobals();
+    $server = $request->server->all();
+    if (basename($server['SCRIPT_FILENAME']) != basename($server['SCRIPT_NAME'])) {
+      // We need this for when the test is executed by run-tests.sh.
+      // @todo Remove this once run-tests.sh has been converted to use a Request
+      //   object.
+      $server['SCRIPT_FILENAME'] = $server['SCRIPT_NAME'];
+      $base_path = ltrim($server['REQUEST_URI'], '/');
+    }
+    else {
+      $base_path = $request->getBasePath();
+    }
+    $sub_request = Request::create($base_path . '/user', 'GET', $request->query->all(), $request->cookies->all(), [], $server);
+    return $this->httpKernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST);
+  }
+
+}

core/modules/language/tests/language_test/src/Entity/NoLanguageEntityTest.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\language_test\Entity;
+
+use Drupal\Core\Entity\ContentEntityBase;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Field\BaseFieldDefinition;
+
+/**
+ * Defines the test entity class.
+ *
+ * @ContentEntityType(
+ *   id = "no_language_entity_test",
+ *   label = @Translation("Test entity without language support"),
+ *   handlers = {
+ *     "views_data" = "Drupal\entity_test\EntityTestViewsData"
+ *   },
+ *   base_table = "no_language_entity_test",
+ *   persistent_cache = FALSE,
+ *   entity_keys = {
+ *     "id" = "id",
+ *     "uuid" = "uuid",
+ *   },
+ * )
+ */
+class NoLanguageEntityTest extends ContentEntityBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
+    $fields['id'] = BaseFieldDefinition::create('integer')
+      ->setLabel(t('ID'))
+      ->setDescription(t('The ID of the test entity.'))
+      ->setReadOnly(TRUE)
+      ->setSetting('unsigned', TRUE);
+
+    $fields['uuid'] = BaseFieldDefinition::create('uuid')
+      ->setLabel(t('UUID'))
+      ->setDescription(t('The UUID of the test entity.'))
+      ->setReadOnly(TRUE);
+
+    return $fields;
+  }
+
+}

core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTest.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Drupal\language_test\Plugin\LanguageNegotiation;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+use Drupal\language\LanguageNegotiationMethodBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Class for identifying language from a selected language.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationTest::METHOD_ID,
+  name: new TranslatableMarkup('Test'),
+  types: [LanguageInterface::TYPE_CONTENT,
+    'test_language_type',
+    'fixed_test_language_type',
+  ],
+  weight: -10,
+  description: new TranslatableMarkup('This is a test language negotiation method.'),
+)]
+class LanguageNegotiationTest extends LanguageNegotiationMethodBase {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'test_language_negotiation_method';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLangcode(?Request $request = NULL) {
+    return 'it';
+  }
+
+}

core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestTs.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\language_test\Plugin\LanguageNegotiation;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\language\Attribute\LanguageNegotiation;
+
+/**
+ * Class for identifying language from a selected language.
+ */
+#[LanguageNegotiation(
+  id: LanguageNegotiationTestTs::METHOD_ID,
+  name: new TranslatableMarkup('Type-specific test'),
+  types: ['test_language_type'],
+  weight: -10,
+  description: new TranslatableMarkup('This is a test language negotiation method.'),
+)]
+class LanguageNegotiationTestTs extends LanguageNegotiationTest {
+
+  /**
+   * The language negotiation method id.
+   */
+  const METHOD_ID = 'test_language_negotiation_method_ts';
+
+}

core/modules/language/tests/src/Functional/AdminPathEntityConverterLanguageTest.php

@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Test administration path based conversion of entities.
+ *
+ * @group language
+ */
+class AdminPathEntityConverterLanguageTest extends BrowserTestBase {
+
+  protected static $modules = ['language', 'language_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $permissions = [
+      'access administration pages',
+      'administer site configuration',
+    ];
+    $this->drupalLogin($this->drupalCreateUser($permissions));
+    ConfigurableLanguage::createFromLangcode('es')->save();
+  }
+
+  /**
+   * Tests the translated and untranslated config entities are loaded properly.
+   */
+  public function testConfigUsingCurrentLanguage(): void {
+    \Drupal::languageManager()
+      ->getLanguageConfigOverride('es', 'language.entity.es')
+      ->set('label', 'Español')
+      ->save();
+
+    $this->drupalGet('es/admin/language_test/entity_using_current_language/es');
+    $this->assertSession()->pageTextNotContains('Loaded Spanish.');
+    $this->assertSession()->pageTextContains('Loaded Español.');
+
+    $this->drupalGet('es/admin/language_test/entity_using_original_language/es');
+    $this->assertSession()->pageTextContains('Loaded Spanish.');
+    $this->assertSession()->pageTextNotContains('Loaded Español.');
+  }
+
+}

core/modules/language/tests/src/Functional/ConfigurableLanguageManagerTest.php

@@ -0,0 +1,281 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\node\Entity\Node;
+use Drupal\node\Entity\NodeType;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\WaitTerminateTestTrait;
+
+// cspell:ignore funciona
+
+/**
+ * Tests Language Negotiation.
+ *
+ * Uses different negotiators for content and interface.
+ *
+ * @group language
+ */
+class ConfigurableLanguageManagerTest extends BrowserTestBase {
+
+  use WaitTerminateTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'content_translation',
+    'node',
+    'locale',
+    'block',
+    'system',
+    'user',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // The \Drupal\locale\LocaleTranslation service clears caches after the
+    // response is flushed to the client. We use WaitTerminateTestTrait to wait
+    // for Drupal to perform its termination work before continuing.
+    $this->setWaitForTerminate();
+
+    /** @var \Drupal\user\UserInterface $user */
+    $user = $this->createUser([], '', TRUE);
+    $this->drupalLogin($user);
+    ConfigurableLanguage::createFromLangcode('es')->save();
+
+    // Create a page node type and make it translatable.
+    NodeType::create([
+      'type' => 'page',
+      'name' => 'Page',
+    ])->save();
+
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'page');
+    $config->setDefaultLangcode('en')
+      ->setLanguageAlterable(TRUE)
+      ->save();
+
+    // Create a Node with title 'English' and translate it to Spanish.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'English',
+    ]);
+    $node->save();
+    $node->addTranslation('es', ['title' => 'Español']);
+    $node->save();
+
+    // Enable both language_interface and language_content language negotiation.
+    \Drupal::getContainer()->get('language_negotiator')->updateConfiguration([
+      'language_interface',
+      'language_content',
+    ]);
+
+    // Set the preferred language of the user for admin pages to English.
+    $user->set('preferred_admin_langcode', 'en')->save();
+
+    // Make sure node edit pages are administration pages.
+    $this->config('node.settings')->set('use_admin_theme', '1')->save();
+    $this->container->get('router.builder')->rebuild();
+
+    // Place a Block with a translatable string on the page.
+    $this->placeBlock('system_powered_by_block', ['region' => 'content']);
+
+    // Load the Spanish Node page once, to register the translatable string.
+    $this->drupalGet('/es/node/1');
+
+    // Translate the Powered by string.
+    /** @var \Drupal\locale\StringStorageInterface $string_storage */
+    $string_storage = \Drupal::getContainer()->get('locale.storage');
+    $source = $string_storage->findString(['source' => 'Powered by <a href=":poweredby">Drupal</a>']);
+    $string_storage->createTranslation([
+      'lid' => $source->lid,
+      'language' => 'es',
+      'translation' => 'Funciona con ...',
+    ])->save();
+    // Invalidate caches so that the new translation will be used.
+    Cache::invalidateTags(['rendered', 'locale']);
+  }
+
+  /**
+   * Tests translation with URL and Preferred Admin Language negotiators.
+   *
+   * The interface language uses the preferred language for admin pages of the
+   * user and after that the URL. The Content uses just the URL.
+   */
+  public function testUrlContentTranslationWithPreferredAdminLanguage(): void {
+    $assert_session = $this->assertSession();
+    // Set the interface language to use the preferred administration language
+    // and then the URL.
+    /** @var \Drupal\language\LanguageNegotiatorInterface $language_negotiator */
+    $language_negotiator = \Drupal::getContainer()->get('language_negotiator');
+    $language_negotiator->saveConfiguration('language_interface', [
+      'language-user-admin' => 1,
+      'language-url' => 2,
+      'language-selected' => 3,
+    ]);
+    // Set Content Language Negotiator to use just the URL.
+    $language_negotiator->saveConfiguration('language_content', [
+      'language-url' => 4,
+      'language-selected' => 5,
+    ]);
+
+    // See if the full view of the node in english is present and the
+    // string in the Powered By Block is in English.
+    $this->drupalGet('/node/1');
+    $assert_session->pageTextContains('English');
+    $assert_session->pageTextContains('Powered by');
+
+    // Load the spanish node page again and see if both the node and the string
+    // are translated.
+    $this->drupalGet('/es/node/1');
+    $assert_session->pageTextContains('Español');
+    $assert_session->pageTextContains('Funciona con');
+    $assert_session->pageTextNotContains('Powered by');
+
+    // Check if the Powered by string is shown in English on an
+    // administration page, and the node content is shown in Spanish.
+    $this->drupalGet('/es/node/1/edit');
+    $assert_session->pageTextContains('Español');
+    $assert_session->pageTextContains('Powered by');
+    $assert_session->pageTextNotContains('Funciona con');
+  }
+
+  /**
+   * Tests translation with URL and Session Language Negotiators.
+   */
+  public function testUrlContentTranslationWithSessionLanguage(): void {
+    $assert_session = $this->assertSession();
+    /** @var \Drupal\language\LanguageNegotiatorInterface $language_negotiator */
+    $language_negotiator = \Drupal::getContainer()->get('language_negotiator');
+    // Set Interface Language Negotiator to Session.
+    $language_negotiator->saveConfiguration('language_interface', [
+      'language-session' => 1,
+      'language-url' => 2,
+      'language-selected' => 3,
+    ]);
+
+    // Set Content Language Negotiator to URL.
+    $language_negotiator->saveConfiguration('language_content', [
+      'language-url' => 4,
+      'language-selected' => 5,
+    ]);
+
+    // See if the full view of the node in english is present and the
+    // string in the Powered By Block is in English.
+    $this->drupalGet('/node/1');
+    $assert_session->pageTextContains('English');
+    $assert_session->pageTextContains('Powered by');
+
+    // The language session variable has not been set yet, so
+    // The string should be in Spanish.
+    $this->drupalGet('/es/node/1');
+    $assert_session->pageTextContains('Español');
+    $assert_session->pageTextNotContains('Powered by');
+    $assert_session->pageTextContains('Funciona con');
+
+    // Set the session language to Spanish but load the English node page.
+    $this->drupalGet('/node/1', ['query' => ['language' => 'es']]);
+    $assert_session->pageTextContains('English');
+    $assert_session->pageTextNotContains('Español');
+    $assert_session->pageTextContains('Funciona con');
+    $assert_session->pageTextNotContains('Powered by');
+
+    // Set the session language to English but load the node page in Spanish.
+    $this->drupalGet('/es/node/1', ['query' => ['language' => 'en']]);
+    $assert_session->pageTextNotContains('English');
+    $assert_session->pageTextContains('Español');
+    $assert_session->pageTextNotContains('Funciona con');
+    $assert_session->pageTextContains('Powered by');
+  }
+
+  /**
+   * Tests translation of the user profile edit form.
+   *
+   * The user profile edit form is a special case when used with the preferred
+   * admin language negotiator because of the recursive way that the negotiator
+   * is called.
+   */
+  public function testUserProfileTranslationWithPreferredAdminLanguage(): void {
+    $assert_session = $this->assertSession();
+    // Set the interface language to use the preferred administration language.
+    /** @var \Drupal\language\LanguageNegotiatorInterface $language_negotiator */
+    $language_negotiator = \Drupal::getContainer()->get('language_negotiator');
+    $language_negotiator->saveConfiguration('language_interface', [
+      'language-user-admin' => 1,
+      'language-selected' => 2,
+    ]);
+
+    // Create a field on the user entity.
+    $field_name = $this->randomMachineName();
+    $label = $this->randomMachineName();
+    $field_label_en = "English $label";
+    $field_label_es = "Español $label";
+
+    $field_storage = FieldStorageConfig::create([
+      'field_name' => $field_name,
+      'entity_type' => 'user',
+      'type' => 'string',
+    ]);
+    $field_storage->save();
+
+    $instance = FieldConfig::create([
+      'field_storage' => $field_storage,
+      'bundle' => 'user',
+      'label' => $field_label_en,
+    ]);
+    $instance->save();
+
+    // Add a Spanish translation.
+    \Drupal::languageManager()
+      ->getLanguageConfigOverride('es', "field.field.user.user.$field_name")
+      ->set('label', $field_label_es)
+      ->save();
+
+    // Add the new field to the edit form.
+    EntityFormDisplay::create([
+      'targetEntityType' => 'user',
+      'bundle' => 'user',
+      'mode' => 'default',
+      'status' => TRUE,
+    ])
+      ->setComponent($field_name, [
+        'type' => 'string_textfield',
+      ])
+      ->save();
+
+    $user_id = \Drupal::currentUser()->id();
+    $this->drupalGet("/user/$user_id/edit");
+    // Admin language choice is "No preference" so we should get the default.
+    $assert_session->pageTextContains($field_label_en);
+    $assert_session->pageTextNotContains($field_label_es);
+
+    // Set admin language to Spanish.
+    $this->submitForm(['edit-preferred-admin-langcode' => 'es'], 'edit-submit');
+    $assert_session->pageTextContains($field_label_es);
+    $assert_session->pageTextNotContains($field_label_en);
+
+    // Set admin language to English.
+    $this->submitForm(['edit-preferred-admin-langcode' => 'en'], 'edit-submit');
+    $assert_session->pageTextContains($field_label_en);
+    $assert_session->pageTextNotContains($field_label_es);
+  }
+
+}

core/modules/language/tests/src/Functional/EntityTypeWithoutLanguageFormTest.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests entity type without language support.
+ *
+ * This is to ensure that an entity type without language support can not
+ * enable the language select from the content language settings page.
+ *
+ * @covers \Drupal\language\Form\ContentLanguageSettingsForm
+ * @group language
+ */
+class EntityTypeWithoutLanguageFormTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'language_test',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create and log in administrative user.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+    ]);
+    $this->drupalLogin($admin_user);
+  }
+
+  /**
+   * Tests configuration options with an entity without language definition.
+   */
+  public function testEmptyLangcode(): void {
+    // Assert that we can not enable language select from
+    // content language settings page.
+    $this->drupalGet('admin/config/regional/content-language');
+    $this->assertSession()->fieldNotExists('entity_types[no_language_entity_test]');
+  }
+
+}

core/modules/language/tests/src/Functional/GenericTest.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\system\Functional\Module\GenericModuleTestBase;
+
+/**
+ * Generic module test for language.
+ *
+ * @group language
+ */
+class GenericTest extends GenericModuleTestBase {}

core/modules/language/tests/src/Functional/LanguageBlockSettingsVisibilityTest.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests that the language settings on block config appears correctly.
+ *
+ * @group language
+ */
+class LanguageBlockSettingsVisibilityTest extends BrowserTestBase {
+
+  protected static $modules = ['block', 'language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  public function testUnnecessaryLanguageSettingsVisibility(): void {
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'administer blocks',
+    ]);
+    $this->drupalLogin($admin_user);
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'hu'], 'Add language');
+    $this->drupalGet('admin/structure/block/add/system_menu_block:admin/stark');
+    $this->assertSession()->fieldNotExists("edit-visibility-language-langcodes-und");
+    $this->assertSession()->fieldNotExists("edit-visibility-language-langcodes-zxx");
+    $this->assertSession()->fieldExists("edit-visibility-language-langcodes-en");
+    $this->assertSession()->fieldExists("edit-visibility-language-langcodes-hu");
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageBreadcrumbTest.php

@@ -0,0 +1,75 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\system\Functional\Menu\AssertBreadcrumbTrait;
+
+/**
+ * Tests breadcrumbs functionality.
+ *
+ * @group Menu
+ */
+class LanguageBreadcrumbTest extends BrowserTestBase {
+
+  use AssertBreadcrumbTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'block', 'filter'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->drupalPlaceBlock('system_breadcrumb_block');
+    ConfigurableLanguage::createFromLangcode('de')->save();
+    ConfigurableLanguage::createFromLangcode('gsw-berne')->save();
+  }
+
+  /**
+   * Tests breadcrumbs with URL prefixes.
+   */
+  public function testBreadCrumbs(): void {
+    // /user/login is the default frontpage which only works for an anonymous
+    // user. Access the frontpage in different languages, ensure that no
+    // breadcrumb is displayed.
+    $this->assertBreadcrumb('user/login', []);
+    $this->assertBreadcrumb('de/user/login', []);
+    $this->assertBreadcrumb('gsw-berne/user/login', []);
+
+    $admin_user = $this->drupalCreateUser(['access administration pages', 'administer blocks']);
+    $this->drupalLogin($admin_user);
+
+    // Use administration routes to assert that breadcrumb is displayed
+    // correctly on pages other than the frontpage.
+    $this->assertBreadcrumb('admin', [
+      '' => 'Home',
+    ]);
+    $this->assertBreadcrumb('de/admin', [
+      'de' => 'Home',
+    ]);
+
+    $this->assertBreadcrumb('admin/structure', [
+      '' => 'Home',
+      'admin' => 'Administration',
+    ]);
+    $this->assertBreadcrumb('de/admin/structure', [
+      'de' => 'Home',
+      'de/admin' => 'Administration',
+    ]);
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageBrowserDetectionAcceptLanguageTest.php

@@ -0,0 +1,129 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests browser language detection with different accept-language headers.
+ *
+ * @group language
+ */
+class LanguageBrowserDetectionAcceptLanguageTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'locale',
+    'content_translation',
+    'system_test',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    // User to manage languages.
+    $admin = $this->drupalCreateUser([], NULL, TRUE);
+    $this->drupalLogin($admin);
+
+    // Create FR.
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+    // Set language detection to URL and browser detection.
+    $this->drupalGet('/admin/config/regional/language/detection');
+    $this->submitForm([
+      'language_interface[enabled][language-url]' => TRUE,
+      'language_interface[enabled][language-browser]' => TRUE,
+      'language_interface[enabled][language-selected]' => TRUE,
+    ], 'Save settings');
+
+    // Set prefixes to en and fr.
+    $this->drupalGet('/admin/config/regional/language/detection/url');
+    $this->submitForm([
+      'prefix[en]' => 'en',
+      'prefix[fr]' => 'fr',
+    ], 'Save configuration');
+    // Add language codes to browser detection.
+    $this->drupalGet('/admin/config/regional/language/detection/browser');
+    $this->submitForm([
+      'new_mapping[browser_langcode]' => 'fr',
+      'new_mapping[drupal_langcode]' => 'fr',
+    ], 'Save configuration');
+    $this->drupalGet('/admin/config/regional/language/detection/browser');
+    $this->submitForm([
+      'new_mapping[browser_langcode]' => 'en',
+      'new_mapping[drupal_langcode]' => 'en',
+    ], 'Save configuration');
+    $this->drupalGet('/admin/config/regional/language/detection/selected');
+    $this->submitForm(['edit-selected-langcode' => 'en'], 'Save configuration');
+
+    $this->drupalLogout();
+  }
+
+  /**
+   * Tests with browsers with and without Accept-Language header.
+   */
+  public function testAcceptLanguageEmptyDefault(): void {
+
+    // Check correct headers.
+    $this->drupalGet('/en/system-test/echo/language test', [], ['Accept-Language' => 'en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
+
+    $this->drupalGet('/fr/system-test/echo/language test', [], ['Accept-Language' => 'en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
+    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
+
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check with UK browser.
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en-UK,en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check with french browser.
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'fr-FR,fr']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check with browser without language settings - should return fallback language.
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => '']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check with french browser again.
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'fr-FR,fr']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check with UK browser.
+    $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en-UK,en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
+
+    // Check if prefixed URLs are still cached.
+    $this->drupalGet('/en/system-test/echo/language test', [], ['Accept-Language' => 'en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
+    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
+
+    $this->drupalGet('/fr/system-test/echo/language test', [], ['Accept-Language' => 'en']);
+    $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
+    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageBrowserDetectionTest.php

@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests browser language detection.
+ *
+ * @group language
+ */
+class LanguageBrowserDetectionTest extends BrowserTestBase {
+
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests mappings between browser language codes and Drupal language codes.
+   */
+  public function testUIBrowserLanguageMappings(): void {
+    // User to manage languages.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Check that the configure link exists.
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->assertSession()->linkByHrefExists('admin/config/regional/language/detection/browser');
+
+    // Check that defaults are loaded from language.mappings.yml.
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->assertSession()->fieldValueEquals('edit-mappings-zh-cn-browser-langcode', 'zh-cn');
+    $this->assertSession()->fieldValueEquals('edit-mappings-zh-cn-drupal-langcode', 'zh-hans');
+
+    // Delete zh-cn language code.
+    $browser_langcode = 'zh-cn';
+    $this->drupalGet('admin/config/regional/language/detection/browser/delete/' . $browser_langcode);
+    $this->assertSession()->pageTextContains("Are you sure you want to delete {$browser_langcode}?");
+
+    // Confirm the delete.
+    $edit = [];
+    $this->drupalGet('admin/config/regional/language/detection/browser/delete/' . $browser_langcode);
+    $this->submitForm($edit, 'Confirm');
+
+    $this->assertSession()->statusMessageContains("The mapping for the {$browser_langcode} browser language code has been deleted.", 'status');
+
+    // Check we went back to the browser negotiation mapping overview.
+    $this->assertSession()->addressEquals(Url::fromRoute('language.negotiation_browser'));
+    // Check that Chinese browser language code no longer exists.
+    $this->assertSession()->fieldNotExists('edit-mappings-zh-cn-browser-langcode');
+
+    // Add a new custom mapping.
+    $edit = [
+      'new_mapping[browser_langcode]' => 'xx',
+      'new_mapping[drupal_langcode]' => 'en',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->addressEquals(Url::fromRoute('language.negotiation_browser'));
+    $this->assertSession()->fieldValueEquals('edit-mappings-xx-browser-langcode', 'xx');
+    $this->assertSession()->fieldValueEquals('edit-mappings-xx-drupal-langcode', 'en');
+
+    // Add the same custom mapping again.
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('Browser language codes must be unique.', 'error');
+
+    // Change browser language code of our custom mapping to zh-sg.
+    $edit = [
+      'mappings[xx][browser_langcode]' => 'zh-sg',
+      'mappings[xx][drupal_langcode]' => 'en',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('Browser language codes must be unique.', 'error');
+
+    // Change Drupal language code of our custom mapping to zh-hans.
+    $edit = [
+      'mappings[xx][browser_langcode]' => 'xx',
+      'mappings[xx][drupal_langcode]' => 'zh-hans',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/browser');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->addressEquals(Url::fromRoute('language.negotiation_browser'));
+    $this->assertSession()->fieldValueEquals('edit-mappings-xx-browser-langcode', 'xx');
+    $this->assertSession()->fieldValueEquals('edit-mappings-xx-drupal-langcode', 'zh-hans');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageConfigInstallOverrideExistingTest.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Ensures the language config is installed but not altered on install.
+ *
+ * @group language
+ */
+class LanguageConfigInstallOverrideExistingTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $profile = 'test_language_negotiation';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests when language config is installed existing config is not overridden.
+   */
+  public function testLanguageConfigInstallOverrideExisting(): void {
+    /** @var \Drupal\Core\Config\StorageInterface $storage */
+    $storage = $this->container->get('config.storage');
+    $config = $this->config('language.types');
+
+    // The negotiation methods that have been removed should be disabled after
+    // purging if not avoided in language_modules_installed().
+    $language_types_data = $storage->read('language.types');
+    $this->assertTrue(isset($language_types_data['negotiation']['language_content']['enabled']['test_language_negotiation_method']));
+    $this->assertTrue(isset($language_types_data['negotiation']['language_content']['enabled']['language-selected']));
+
+    $this->assertEquals(-10, $config->get('negotiation.language_content.enabled.test_language_negotiation_method'));
+    $this->assertEquals(12, $config->get('negotiation.language_content.enabled.language-selected'));
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageConfigOverrideImportTest.php

@@ -0,0 +1,109 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Config\ConfigCollectionEvents;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Ensures the language config overrides can be synchronized.
+ *
+ * @group language
+ */
+class LanguageConfigOverrideImportTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'config',
+    'locale',
+    'config_translation',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests that language can be enabled and overrides are created during a sync.
+   */
+  public function testConfigOverrideImport(): void {
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+    /** @var \Drupal\Core\Config\StorageInterface $sync */
+    $sync = \Drupal::service('config.storage.sync');
+    $this->copyConfig(\Drupal::service('config.storage'), $sync);
+
+    // Uninstall the language module and its dependencies so we can test
+    // enabling the language module and creating overrides at the same time
+    // during a configuration synchronization.
+    \Drupal::service('module_installer')->uninstall(['language']);
+    // Ensure that the current site has no overrides registered to the
+    // ConfigFactory.
+    $this->rebuildContainer();
+
+    /** @var \Drupal\Core\Config\StorageInterface $override_sync */
+    $override_sync = $sync->createCollection('language.fr');
+    // Create some overrides in sync.
+    $override_sync->write('system.site', ['name' => 'FR default site name']);
+    $override_sync->write('system.maintenance', ['message' => 'FR message: @site is currently under maintenance. We should be back shortly. Thank you for your patience']);
+
+    $this->configImporter()->import();
+    $this->rebuildContainer();
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('fr', 'system.site');
+    $this->assertEquals('FR default site name', $override->get('name'));
+    $this->drupalGet('fr');
+    $this->assertSession()->pageTextContains('FR default site name');
+    $this->drupalLogin($this->drupalCreateUser([
+      'translate configuration',
+    ]));
+    $this->drupalGet('admin/config/development/maintenance/translate/fr/edit');
+    $this->assertSession()->pageTextContains('FR message: @site is currently under maintenance. We should be back shortly. Thank you for your patience');
+  }
+
+  /**
+   * Tests that configuration events are not fired during a sync of overrides.
+   */
+  public function testConfigOverrideImportEvents(): void {
+    // Enable the config_events_test module so we can record events occurring.
+    \Drupal::service('module_installer')->install(['config_events_test']);
+    $this->rebuildContainer();
+
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+
+    /** @var \Drupal\Core\Config\StorageInterface $sync */
+    $sync = \Drupal::service('config.storage.sync');
+    $this->copyConfig(\Drupal::service('config.storage'), $sync);
+
+    /** @var \Drupal\Core\Config\StorageInterface $override_sync */
+    $override_sync = $sync->createCollection('language.fr');
+    // Create some overrides in sync.
+    $override_sync->write('system.site', ['name' => 'FR default site name']);
+    \Drupal::state()->set('config_events_test.event', FALSE);
+
+    $this->configImporter()->import();
+    $this->rebuildContainer();
+
+    // Test that no config save event has been fired during the import because
+    // language configuration overrides do not fire events.
+    $event_recorder = \Drupal::state()->get('config_events_test.event', FALSE);
+    $this->assertSame([
+      'event_name' => ConfigCollectionEvents::SAVE_IN_COLLECTION,
+      'current_config_data' => ['name' => 'FR default site name'],
+      'original_config_data' => [],
+      'raw_config_data' => ['name' => 'FR default site name'],
+    ], $event_recorder);
+
+    $this->drupalGet('fr');
+    $this->assertSession()->pageTextContains('FR default site name');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageConfigSchemaTest.php

@@ -0,0 +1,78 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\SchemaCheckTestTrait;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Ensures the language config schema is correct.
+ *
+ * @group language
+ */
+class LanguageConfigSchemaTest extends BrowserTestBase {
+
+  use SchemaCheckTestTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'menu_link_content'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * A user with administrative permissions.
+   *
+   * @var \Drupal\user\UserInterface
+   */
+  protected $adminUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create user.
+    $this->adminUser = $this->drupalCreateUser(['administer languages']);
+    $this->drupalLogin($this->adminUser);
+  }
+
+  /**
+   * Tests whether the language config schema is valid.
+   */
+  public function testValidLanguageConfigSchema(): void {
+    // Make sure no language configuration available by default.
+    $config_data = $this->config('language.settings')->get();
+    $this->assertEmpty($config_data);
+
+    $settings_path = 'admin/config/regional/content-language';
+
+    // Enable translation for menu link.
+    $edit['entity_types[menu_link_content]'] = TRUE;
+    $edit['settings[menu_link_content][menu_link_content][settings][language][language_alterable]'] = TRUE;
+
+    // Enable translation for user.
+    $edit['entity_types[user]'] = TRUE;
+    $edit['settings[user][user][settings][language][language_alterable]'] = TRUE;
+    $edit['settings[user][user][settings][language][langcode]'] = 'en';
+
+    $this->drupalGet($settings_path);
+    $this->submitForm($edit, 'Save configuration');
+
+    $config_data = $this->config('language.content_settings.menu_link_content.menu_link_content');
+    // Make sure configuration saved correctly.
+    $this->assertTrue($config_data->get('language_alterable'));
+
+    $this->assertConfigSchema(\Drupal::service('config.typed'), $config_data->getName(), $config_data->get());
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageConfigurationElementTest.php

@@ -0,0 +1,284 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\taxonomy\Entity\Vocabulary;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the features of the language configuration element field.
+ *
+ * @group language
+ */
+class LanguageConfigurationElementTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'taxonomy',
+    'node',
+    'language',
+    'language_elements_test',
+    'field_ui',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $user = $this->drupalCreateUser([
+      'access administration pages',
+      'administer languages',
+      'administer content types',
+    ]);
+    $this->drupalLogin($user);
+  }
+
+  /**
+   * Tests the language settings have been saved.
+   */
+  public function testLanguageConfigurationElement(): void {
+    $this->drupalGet('language-tests/language_configuration_element');
+    $edit['lang_configuration[langcode]'] = 'current_interface';
+    $edit['lang_configuration[language_alterable]'] = FALSE;
+    $this->submitForm($edit, 'Save');
+    $lang_conf = ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'some_bundle');
+
+    // Check that the settings have been saved.
+    $this->assertEquals('current_interface', $lang_conf->getDefaultLangcode());
+    $this->assertFalse($lang_conf->isLanguageAlterable());
+    $this->drupalGet('language-tests/language_configuration_element');
+    $this->assertTrue($this->assertSession()->optionExists('edit-lang-configuration-langcode', 'current_interface')->isSelected());
+    $this->assertSession()->checkboxNotChecked('edit-lang-configuration-language-alterable');
+
+    // Reload the page and save again.
+    $this->drupalGet('language-tests/language_configuration_element');
+    $edit['lang_configuration[langcode]'] = 'authors_default';
+    $edit['lang_configuration[language_alterable]'] = TRUE;
+    $this->submitForm($edit, 'Save');
+    $lang_conf = ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'some_bundle');
+
+    // Check that the settings have been saved.
+    $this->assertEquals('authors_default', $lang_conf->getDefaultLangcode());
+    $this->assertTrue($lang_conf->isLanguageAlterable());
+    $this->drupalGet('language-tests/language_configuration_element');
+    $this->assertTrue($this->assertSession()->optionExists('edit-lang-configuration-langcode', 'authors_default')->isSelected());
+    $this->assertSession()->checkboxChecked('edit-lang-configuration-language-alterable');
+
+    // Test if content type settings have been saved.
+    $edit = [
+      'name' => 'Page',
+      'type' => 'page',
+      'language_configuration[langcode]' => 'authors_default',
+      'language_configuration[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet('admin/structure/types/add');
+    $this->submitForm($edit, 'Save and manage fields');
+
+    // Make sure the settings are saved when creating the content type.
+    $this->drupalGet('admin/structure/types/manage/page');
+    $this->assertTrue($this->assertSession()->optionExists('edit-language-configuration-langcode', 'authors_default')->isSelected());
+    $this->assertSession()->checkboxChecked('edit-language-configuration-language-alterable');
+
+  }
+
+  /**
+   * Tests that the language_get_default_langcode() returns the correct values.
+   */
+  public function testDefaultLangcode(): void {
+    // Add some custom languages.
+    foreach (['aa', 'bb', 'cc'] as $language_code) {
+      ConfigurableLanguage::create([
+        'id' => $language_code,
+        'label' => $this->randomMachineName(),
+      ])->save();
+    }
+
+    // Ensure the bundles under test exist, to avoid config validation errors.
+    entity_test_create_bundle('custom_bundle');
+    entity_test_create_bundle('some_bundle');
+
+    // Fixed language.
+    ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'custom_bundle')
+      ->setLanguageAlterable(TRUE)
+      ->setDefaultLangcode('bb')
+      ->save();
+
+    $langcode = language_get_default_langcode('entity_test', 'custom_bundle');
+    $this->assertEquals('bb', $langcode);
+
+    // Current interface.
+    ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'custom_bundle')
+      ->setLanguageAlterable(TRUE)
+      ->setDefaultLangcode('current_interface')
+      ->save();
+
+    $langcode = language_get_default_langcode('entity_test', 'custom_bundle');
+    $language_interface = \Drupal::languageManager()->getCurrentLanguage();
+    $this->assertEquals($langcode, $language_interface->getId());
+
+    // Site's default.
+    $old_default = \Drupal::languageManager()->getDefaultLanguage();
+    // Ensure the language entity default value is correct.
+    $configurable_language = ConfigurableLanguage::load($old_default->getId());
+    $this->assertTrue($configurable_language->isDefault(), 'The en language entity is flagged as the default language.');
+
+    $this->config('system.site')->set('default_langcode', 'cc')->save();
+    ContentLanguageSettings::loadByEntityTypeBundle('entity_test', 'custom_bundle')
+      ->setLanguageAlterable(TRUE)
+      ->setDefaultLangcode(LanguageInterface::LANGCODE_SITE_DEFAULT)
+      ->save();
+    $langcode = language_get_default_langcode('entity_test', 'custom_bundle');
+    $this->assertEquals('cc', $langcode);
+
+    // Ensure the language entity default value is correct.
+    $configurable_language = ConfigurableLanguage::load($old_default->getId());
+    $this->assertFalse($configurable_language->isDefault(), 'The en language entity is not flagged as the default language.');
+    $configurable_language = ConfigurableLanguage::load('cc');
+    // Check calling the
+    // \Drupal\language\ConfigurableLanguageInterface::isDefault() method
+    // directly.
+    $this->assertTrue($configurable_language->isDefault(), 'The cc language entity is flagged as the default language.');
+
+    // Check the default value of a language field when authors preferred option
+    // is selected.
+    // First create a user, then assign a langcode.
+    $some_user = $this->drupalCreateUser();
+    $some_user->preferred_langcode = 'bb';
+    $some_user->save();
+    $this->drupalLogin($some_user);
+    ContentLanguageSettings::create([
+      'target_entity_type_id' => 'entity_test',
+      'target_bundle' => 'some_bundle',
+    ])->setLanguageAlterable(TRUE)
+      ->setDefaultLangcode('authors_default')
+      ->save();
+
+    $this->drupalGet('language-tests/language_configuration_element_test');
+    $this->assertTrue($this->assertSession()->optionExists('edit-langcode', 'bb')->isSelected());
+  }
+
+  /**
+   * Tests that the configuration is retained when the node type is updated.
+   */
+  public function testNodeTypeUpdate(): void {
+    // Create the article content type first if the profile used is not the
+    // standard one.
+    if ($this->profile != 'standard') {
+      $this->drupalCreateContentType(['type' => 'article', 'name' => 'Article']);
+    }
+    $admin_user = $this->drupalCreateUser(['administer content types']);
+    $this->drupalLogin($admin_user);
+    $edit = [
+      'language_configuration[langcode]' => 'current_interface',
+      'language_configuration[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet('admin/structure/types/manage/article');
+    $this->submitForm($edit, 'Save');
+    // Check the language default configuration for the articles.
+    $configuration = ContentLanguageSettings::loadByEntityTypeBundle('node', 'article');
+    $uuid = $configuration->uuid();
+    $this->assertEquals('current_interface', $configuration->getDefaultLangcode(), 'The default language configuration has been saved on the Article content type.');
+    $this->assertTrue($configuration->isLanguageAlterable(), 'The alterable language configuration has been saved on the Article content type.');
+    // Update the article content type by changing the title label.
+    $edit = [
+      'title_label' => 'Name',
+    ];
+    $this->drupalGet('admin/structure/types/manage/article');
+    $this->submitForm($edit, 'Save');
+    // Check that we still have the settings for the updated node type.
+    $configuration = ContentLanguageSettings::loadByEntityTypeBundle('node', 'article');
+    $this->assertEquals('current_interface', $configuration->getDefaultLangcode(), 'The default language configuration has been kept on the updated Article content type.');
+    $this->assertTrue($configuration->isLanguageAlterable(), 'The alterable language configuration has been kept on the updated Article content type.');
+    $this->assertEquals($uuid, $configuration->uuid(), 'The language configuration uuid has been kept on the updated Article content type.');
+  }
+
+  /**
+   * Tests the language settings are deleted on bundle delete.
+   */
+  public function testNodeTypeDelete(): void {
+    // Create the article content type first if the profile used is not the
+    // standard one.
+    if ($this->profile != 'standard') {
+      $this->drupalCreateContentType([
+        'type' => 'article',
+        'name' => 'Article',
+      ]);
+    }
+    $admin_user = $this->drupalCreateUser(['administer content types']);
+    $this->drupalLogin($admin_user);
+
+    // Create language configuration for the articles.
+    $edit = [
+      'language_configuration[langcode]' => 'authors_default',
+      'language_configuration[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet('admin/structure/types/manage/article');
+    $this->submitForm($edit, 'Save');
+
+    // Check the language default configuration for articles is present.
+    $configuration = \Drupal::entityTypeManager()->getStorage('language_content_settings')->load('node.article');
+    $this->assertNotEmpty($configuration, 'The language configuration is present.');
+
+    // Delete 'article' bundle.
+    $this->drupalGet('admin/structure/types/manage/article/delete');
+    $this->submitForm([], 'Delete');
+
+    // Check that the language configuration has been deleted.
+    \Drupal::entityTypeManager()->getStorage('language_content_settings')->resetCache();
+    $configuration = \Drupal::entityTypeManager()->getStorage('language_content_settings')->load('node.article');
+    $this->assertNull($configuration, 'The language configuration was deleted after bundle was deleted.');
+  }
+
+  /**
+   * Tests that the configuration is retained when a vocabulary is updated.
+   */
+  public function testTaxonomyVocabularyUpdate(): void {
+    $vocabulary = Vocabulary::create([
+      'name' => 'Country',
+      'vid' => 'country',
+    ]);
+    $vocabulary->save();
+
+    $admin_user = $this->drupalCreateUser(['administer taxonomy']);
+    $this->drupalLogin($admin_user);
+    $edit = [
+      'default_language[langcode]' => 'current_interface',
+      'default_language[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet('admin/structure/taxonomy/manage/country');
+    $this->submitForm($edit, 'Save');
+
+    // Check the language default configuration.
+    $configuration = ContentLanguageSettings::loadByEntityTypeBundle('taxonomy_term', 'country');
+    $uuid = $configuration->uuid();
+    $this->assertEquals('current_interface', $configuration->getDefaultLangcode(), 'The default language configuration has been saved on the Country vocabulary.');
+    $this->assertTrue($configuration->isLanguageAlterable(), 'The alterable language configuration has been saved on the Country vocabulary.');
+    // Update the vocabulary.
+    $edit = [
+      'name' => 'Nation',
+    ];
+    $this->drupalGet('admin/structure/taxonomy/manage/country');
+    $this->submitForm($edit, 'Save');
+    // Check that we still have the settings for the updated vocabulary.
+    $configuration = ContentLanguageSettings::loadByEntityTypeBundle('taxonomy_term', 'country');
+    $this->assertEquals('current_interface', $configuration->getDefaultLangcode(), 'The default language configuration has been kept on the updated Country vocabulary.');
+    $this->assertTrue($configuration->isLanguageAlterable(), 'The alterable language configuration has been kept on the updated Country vocabulary.');
+    $this->assertEquals($uuid, $configuration->uuid(), 'The language configuration uuid has been kept on the updated Country vocabulary.');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageConfigurationTest.php

@@ -0,0 +1,236 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Adds and configures languages to check negotiation changes.
+ *
+ * @group language
+ */
+class LanguageConfigurationTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Functional tests for adding, editing and deleting languages.
+   */
+  public function testLanguageConfiguration(): void {
+    // Ensure the after installing the language module the weight of the English
+    // language is still 0.
+    $this->assertEquals(0, ConfigurableLanguage::load('en')->getWeight(), 'The English language has a weight of 0.');
+
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Check if the Default English language has no path prefix.
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->assertSession()->fieldValueEquals("prefix[en]", '');
+
+    // Check that Add language is a primary button.
+    $this->drupalGet('admin/config/regional/language/add');
+    $button = $this->assertSession()->buttonExists('Add language');
+    $this->assertTrue($button->hasClass("button--primary"));
+
+    // Add predefined language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->submitForm($edit, 'Add language');
+    $this->assertSession()->pageTextContains('French');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+    // Langcode for Languages is always 'en'.
+    $language = $this->config('language.entity.fr')->get();
+    $this->assertEquals('en', $language['langcode']);
+
+    // Check if the Default English language has no path prefix.
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->assertSession()->fieldValueEquals("prefix[en]", '');
+    // Check if French has a path prefix.
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->assertSession()->fieldValueEquals("prefix[fr]", 'fr');
+
+    // Check if we can change the default language.
+    $this->drupalGet('admin/config/regional/language');
+    $this->assertSession()->checkboxChecked('edit-site-default-language-en');
+
+    // Change the default language.
+    $edit = [
+      'site_default_language' => 'fr',
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->rebuildContainer();
+    $this->assertSession()->checkboxChecked('edit-site-default-language-fr');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['langcode' => 'fr']));
+
+    // Check if a valid language prefix is added after changing the default
+    // language.
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->assertSession()->fieldValueEquals("prefix[en]", 'en');
+    // Check if French still has a path prefix.
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->assertSession()->fieldValueEquals("prefix[fr]", 'fr');
+
+    // Check that prefix can be changed.
+    $edit = [
+      'prefix[fr]' => 'french',
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->fieldValueEquals("prefix[fr]", 'french');
+
+    // Check that the prefix can be removed.
+    $edit = [
+      'prefix[fr]' => '',
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageNotContains('The prefix may only be left blank for the selected detection fallback language.');
+
+    // Change default negotiation language.
+    $this->config('language.negotiation')->set('selected_langcode', 'fr')->save();
+    // Check that the prefix of a language that is not the negotiation one
+    // cannot be changed to empty string.
+    $edit = [
+      'prefix[en]' => '',
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The prefix may only be left blank for the selected detection fallback language.', 'error');
+
+    // Check that prefix cannot be changed to contain a slash.
+    $edit = [
+      'prefix[en]' => 'foo/bar',
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The prefix may not contain a slash.', 'error');
+
+    // Remove English language and add a new Language to check if langcode of
+    // Language entity is 'en'.
+    $this->drupalGet('admin/config/regional/language/delete/en');
+    $this->submitForm([], 'Delete');
+    $this->rebuildContainer();
+    $this->assertSession()->statusMessageContains('The English (en) language has been removed.', 'status');
+
+    // Ensure that French language has a weight of 1 after being created through
+    // the UI.
+    $french = ConfigurableLanguage::load('fr');
+    $this->assertEquals(1, $french->getWeight(), 'The French language has a weight of 1.');
+    // Ensure that French language can now have a weight of 0.
+    $french->setWeight(0)->save();
+    $this->assertEquals(0, $french->getWeight(), 'The French language has a weight of 0.');
+    // Ensure that new languages created through the API get a weight of 0.
+    $afrikaans = ConfigurableLanguage::createFromLangcode('af');
+    $afrikaans->save();
+    $this->assertEquals(0, $afrikaans->getWeight(), 'The Afrikaans language has a weight of 0.');
+    // Ensure that a new language can be created with any weight.
+    $arabic = ConfigurableLanguage::createFromLangcode('ar');
+    $arabic->setWeight(4)->save();
+    $this->assertEquals(4, $arabic->getWeight(), 'The Arabic language has a weight of 0.');
+
+    $edit = [
+      'predefined_langcode' => 'de',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    $language = $this->config('language.entity.de')->get();
+    $this->assertEquals('fr', $language['langcode']);
+
+    // Ensure that German language has a weight of 5 after being created through
+    // the UI.
+    $french = ConfigurableLanguage::load('de');
+    $this->assertEquals(5, $french->getWeight(), 'The German language has a weight of 5.');
+  }
+
+  /**
+   * Functional tests for setting system language weight on adding, editing and deleting languages.
+   */
+  public function testLanguageConfigurationWeight(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+    $this->checkConfigurableLanguageWeight();
+
+    // Add predefined language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    $this->checkConfigurableLanguageWeight('after adding new language');
+
+    // Re-ordering languages.
+    $edit = [
+      'languages[en][weight]' => $this->getHighestConfigurableLanguageWeight() + 1,
+    ];
+    $this->drupalGet('admin/config/regional/language');
+    $this->submitForm($edit, 'Save configuration');
+    $this->checkConfigurableLanguageWeight('after re-ordering');
+
+    // Remove predefined language.
+    $this->drupalGet('admin/config/regional/language/delete/fr');
+    $this->submitForm([], 'Delete');
+    $this->checkConfigurableLanguageWeight('after deleting a language');
+  }
+
+  /**
+   * Validates system languages are ordered after configurable languages.
+   *
+   * @param string $state
+   *   (optional) A string for customizing assert messages, containing the
+   *   description of the state of the check, for example: 'after re-ordering'.
+   *   Defaults to 'by default'.
+   */
+  protected function checkConfigurableLanguageWeight($state = 'by default') {
+    // Reset language list.
+    \Drupal::languageManager()->reset();
+    $max_configurable_language_weight = $this->getHighestConfigurableLanguageWeight();
+    foreach (\Drupal::languageManager()->getLanguages(LanguageInterface::STATE_LOCKED) as $locked_language) {
+      $this->assertGreaterThan($max_configurable_language_weight, $locked_language->getWeight(), sprintf('System language %s does not have higher weight than configurable languages %s', $locked_language->getName(), $state));
+    }
+  }
+
+  /**
+   * Helper to get maximum weight of configurable (unlocked) languages.
+   *
+   * @return int
+   *   Maximum weight of configurable languages.
+   */
+  protected function getHighestConfigurableLanguageWeight() {
+    $max_weight = 0;
+
+    $storage = $this->container->get('entity_type.manager')
+      ->getStorage('configurable_language');
+    $storage->resetCache();
+    /** @var \Drupal\Core\Language\LanguageInterface[] $languages */
+    $languages = $storage->loadMultiple();
+    foreach ($languages as $language) {
+      if (!$language->isLocked()) {
+        $max_weight = max($max_weight, $language->getWeight());
+      }
+    }
+
+    return $max_weight;
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageCustomLanguageConfigurationTest.php

@@ -0,0 +1,106 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Url;
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Adds and configures custom languages.
+ *
+ * @group language
+ */
+class LanguageCustomLanguageConfigurationTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Functional tests for adding, editing and deleting languages.
+   */
+  public function testLanguageConfiguration(): void {
+
+    // Create user with permissions to add and remove languages.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Add custom language.
+    $edit = [
+      'predefined_langcode' => 'custom',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    // Test validation on missing values.
+    $this->assertSession()->statusMessageContains('Language code field is required.', 'error');
+    $this->assertSession()->statusMessageContains('Language name field is required.', 'error');
+    $empty_language = new Language();
+    $this->assertSession()->checkboxChecked('edit-direction-' . $empty_language->getDirection());
+    $this->assertSession()->addressEquals(Url::fromRoute('language.add'));
+
+    // Test validation of invalid values.
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => 'white space',
+      'label' => '<strong>evil markup</strong>',
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    $this->assertSession()->statusMessageContains('Language code must be a valid language tag as defined by the W3C.', 'error');
+    $this->assertSession()->linkExists("defined by the W3C");
+    $this->assertSession()->linkByHrefExists("http://www.w3.org/International/articles/language-tags/");
+    $this->assertSession()->statusMessageContains('Language name cannot contain any markup.', 'error');
+    $this->assertSession()->addressEquals(Url::fromRoute('language.add'));
+
+    // Test adding a custom language with a numeric region code.
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => 'es-419',
+      'label' => 'Latin American Spanish',
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->assertSession()->statusMessageContains("The language {$edit['label']} has been created and can now be used.", 'status');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+
+    // Test validation of existing language values.
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => 'de',
+      'label' => 'German',
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+
+    // Add the language the first time.
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->assertSession()->statusMessageContains("The language {$edit['label']} has been created and can now be used.", 'status');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+
+    // Add the language a second time and confirm that this is not allowed.
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->assertSession()->statusMessageContains("The language {$edit['label']} ({$edit['langcode']}) already exists.", 'error');
+    $this->assertSession()->addressEquals(Url::fromRoute('language.add'));
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageEntityFieldAccessHookTest.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests language picker compatibility with hook_entity_field_access.
+ *
+ * @group language
+ */
+class LanguageEntityFieldAccessHookTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'node',
+    'text',
+    'field',
+    'filter',
+    'language',
+    'language_entity_field_access_test',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests compatibility with hook_entity_field_access().
+   */
+  public function testHookEntityFieldAccess(): void {
+    // Create an admin user and do the login.
+    $user = $this->drupalCreateUser([], NULL, TRUE);
+    $this->drupalLogin($user);
+
+    // Assess the field is not visible.
+    $this->drupalGet('node/add/page');
+    $this->assertSession()->fieldNotExists('langcode[0][value]');
+
+    $this->drupalLogout();
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageListModuleInstallTest.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests that the language list is not empty when language is installed.
+ *
+ * @group language
+ */
+class LanguageListModuleInstallTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests enabling Language.
+   */
+  public function testModuleInstallLanguageList(): void {
+    // Since LanguageManager::getLanguages() uses static caches we need to do
+    // this by enabling the module using the UI.
+    $admin_user = $this->drupalCreateUser([
+      'access administration pages',
+      'administer modules',
+    ]);
+    $this->drupalLogin($admin_user);
+    $edit = [];
+    $edit['modules[language][enable]'] = 'language';
+    $this->drupalGet('admin/modules');
+    $this->submitForm($edit, 'Install');
+
+    $this->assertEquals(1, \Drupal::state()->get('language_test.language_count_preinstall', 0), 'Using LanguageManager::getLanguages() returns 1 language during Language installation.');
+
+    // Get updated module list by rebuilding container.
+    $this->rebuildContainer();
+    $this->assertTrue(\Drupal::moduleHandler()->moduleExists('language'), 'Language module is enabled');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageListTest.php

@@ -0,0 +1,234 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Url;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Adds a new language and tests changing its status and the default language.
+ *
+ * @group language
+ */
+class LanguageListTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Functional tests for adding, editing and deleting languages.
+   */
+  public function testLanguageList(): void {
+
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Get the weight of the last language.
+    $languages = \Drupal::service('language_manager')->getLanguages();
+    $last_language_weight = end($languages)->getWeight();
+
+    // Add predefined language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    $this->assertSession()->pageTextContains('French');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+
+    // Get the weight of the last language and check that the weight is one unit
+    // heavier than the last configurable language.
+    $this->rebuildContainer();
+    $languages = \Drupal::service('language_manager')->getLanguages();
+    $last_language = end($languages);
+    $this->assertEquals($last_language_weight + 1, $last_language->getWeight());
+    $this->assertEquals($edit['predefined_langcode'], $last_language->getId());
+
+    // Add custom language.
+    $langcode = 'xx';
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => Language::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+    $this->assertSession()->responseContains('"edit-languages-' . $langcode . '-weight"');
+    $this->assertSession()->pageTextContains($name);
+
+    $language = \Drupal::service('language_manager')->getLanguage($langcode);
+    $english = \Drupal::service('language_manager')->getLanguage('en');
+
+    // Check if we can change the default language.
+    $path = 'admin/config/regional/language';
+    $this->drupalGet($path);
+    $this->assertSession()->checkboxChecked('edit-site-default-language-en');
+    // Change the default language.
+    $edit = [
+      'site_default_language' => $langcode,
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->rebuildContainer();
+    $this->assertSession()->checkboxNotChecked('edit-site-default-language-en');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['language' => $language]));
+
+    // Ensure we can't delete the default language.
+    $this->drupalGet('admin/config/regional/language/delete/' . $langcode);
+    $this->assertSession()->statusCodeEquals(403);
+
+    // Ensure 'Edit' link works.
+    $this->drupalGet('admin/config/regional/language');
+    $this->clickLink('Edit');
+    $this->assertSession()->titleEquals('Edit language | Drupal');
+    // Edit a language.
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'label' => $name,
+    ];
+    $this->drupalGet('admin/config/regional/language/edit/' . $langcode);
+    $this->submitForm($edit, 'Save language');
+    $this->assertSession()->pageTextContains($name);
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['language' => $language]));
+
+    // Change back the default language.
+    $edit = [
+      'site_default_language' => 'en',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save configuration');
+    $this->rebuildContainer();
+    // Ensure 'delete' link works.
+    $this->drupalGet('admin/config/regional/language');
+    $this->clickLink('Delete');
+    $this->assertSession()->pageTextContains('Are you sure you want to delete the language');
+    // Delete a language.
+    $this->drupalGet('admin/config/regional/language/delete/' . $langcode);
+    // First test the 'cancel' link.
+    $this->clickLink('Cancel');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['language' => $english]));
+    $this->assertSession()->pageTextContains($name);
+    // Delete the language for real. This a confirm form, we do not need any
+    // fields changed.
+    $this->drupalGet('admin/config/regional/language/delete/' . $langcode);
+    $this->submitForm([], 'Delete');
+    $this->assertSession()->statusMessageContains("The {$name} ({$langcode}) language has been removed.", 'status');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['language' => $english]));
+    // Verify that language is no longer found.
+    $this->drupalGet('admin/config/regional/language/delete/' . $langcode);
+    $this->assertSession()->statusCodeEquals(404);
+
+    // Delete French.
+    $this->drupalGet('admin/config/regional/language/delete/fr');
+    $this->submitForm([], 'Delete');
+    // Make sure the "language_count" state has been updated correctly.
+    $this->rebuildContainer();
+    $this->assertSession()->statusMessageContains('The French (fr) language has been removed.', 'status');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+    // Verify that language is no longer found.
+    $this->drupalGet('admin/config/regional/language/delete/fr');
+    $this->assertSession()->statusCodeEquals(404);
+    // Make sure the "language_count" state has not changed.
+
+    // Ensure we can delete the English language. Right now English is the only
+    // language so we must add a new language and make it the default before
+    // deleting English.
+    $langcode = 'xx';
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => Language::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+    $this->assertSession()->pageTextContains($name);
+
+    // Check if we can change the default language.
+    $path = 'admin/config/regional/language';
+    $this->drupalGet($path);
+    $this->assertSession()->checkboxChecked('edit-site-default-language-en');
+    // Change the default language.
+    $edit = [
+      'site_default_language' => $langcode,
+    ];
+    $this->submitForm($edit, 'Save configuration');
+    $this->rebuildContainer();
+    $this->assertSession()->checkboxNotChecked('edit-site-default-language-en');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection', [], ['language' => $language]));
+
+    $this->drupalGet('admin/config/regional/language/delete/en');
+    $this->submitForm([], 'Delete');
+    $this->assertSession()->statusMessageContains('The English (en) language has been removed.', 'status');
+    $this->rebuildContainer();
+
+    // Ensure we can't delete a locked language.
+    $this->drupalGet('admin/config/regional/language/delete/und');
+    $this->assertSession()->statusCodeEquals(403);
+
+    // Ensure that NL cannot be set default when it's not available.
+    // First create the NL language.
+    $edit = [
+      'predefined_langcode' => 'nl',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Load the form which has now the additional NL language option.
+    $this->drupalGet('admin/config/regional/language');
+
+    // Delete the NL language in the background.
+    $language_storage = $this->container->get('entity_type.manager')->getStorage('configurable_language');
+    $language_storage->load('nl')->delete();
+
+    $this->submitForm(['site_default_language' => 'nl'], 'Save configuration');
+    $this->assertSession()->statusMessageContains('Selected default language no longer exists.', 'error');
+    $this->assertSession()->checkboxNotChecked('edit-site-default-language-xx');
+  }
+
+  /**
+   * Functional tests for the language states (locked or configurable).
+   */
+  public function testLanguageStates(): void {
+    // Add some languages, and also lock some of them.
+    ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'l1'])->save();
+    ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'l2', 'locked' => TRUE])->save();
+    ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'l3'])->save();
+    ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'l4', 'locked' => TRUE])->save();
+    $expected_locked_languages = ['l4' => 'l4', 'l2' => 'l2', 'und' => 'und', 'zxx' => 'zxx'];
+    $expected_all_languages = ['l4' => 'l4', 'l3' => 'l3', 'l2' => 'l2', 'l1' => 'l1', 'en' => 'en', 'und' => 'und', 'zxx' => 'zxx'];
+    $expected_conf_languages = ['l3' => 'l3', 'l1' => 'l1', 'en' => 'en'];
+
+    $locked_languages = $this->container->get('language_manager')->getLanguages(LanguageInterface::STATE_LOCKED);
+    $this->assertEquals([], array_diff_key($expected_locked_languages, $locked_languages), 'Locked languages loaded correctly.');
+
+    $all_languages = $this->container->get('language_manager')->getLanguages(LanguageInterface::STATE_ALL);
+    $this->assertEquals([], array_diff_key($expected_all_languages, $all_languages), 'All languages loaded correctly.');
+
+    $conf_languages = $this->container->get('language_manager')->getLanguages();
+    $this->assertEquals([], array_diff_key($expected_conf_languages, $conf_languages), 'Configurable languages loaded correctly.');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageLocaleListTest.php

@@ -0,0 +1,94 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Url;
+use Drupal\locale\StringStorageInterface;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore espagnol
+
+/**
+ * Adds a new language with translations and tests language list order.
+ *
+ * @group language
+ */
+class LanguageLocaleListTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'locale'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * @var \Drupal\locale\StringStorageInterface
+   */
+  protected StringStorageInterface $storage;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    // Add a default locale storage for all these tests.
+    $this->storage = $this->container->get('locale.storage');
+  }
+
+  /**
+   * Tests adding, editing, and deleting languages.
+   */
+  public function testLanguageLocaleList(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Add predefined language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    $this->assertSession()->statusMessageContains('The language French has been created and can now be used', 'status');
+    $this->assertSession()->addressEquals(Url::fromRoute('entity.configurable_language.collection'));
+    $this->rebuildContainer();
+
+    // Translate Spanish language to French (Espagnol).
+    $source = $this->storage->createString([
+      'source' => 'Spanish',
+      'context' => '',
+    ])->save();
+    $this->storage->createTranslation([
+      'lid' => $source->lid,
+      'language' => 'fr',
+      'translation' => 'Espagnol',
+    ])->save();
+
+    // Get language list displayed in select list.
+    $this->drupalGet('fr/admin/config/regional/language/add');
+    $options = $this->assertSession()->selectExists('edit-predefined-langcode')->findAll('css', 'option');
+    $options = array_map(function ($item) {
+      return $item->getText();
+    }, $options);
+    // Remove the 'Custom language...' option form the end.
+    array_pop($options);
+    // Order language list.
+    $options_ordered = $options;
+    natcasesort($options_ordered);
+
+    // Check the language list displayed is ordered.
+    $this->assertSame($options, $options_ordered, 'Language list is ordered.');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageNegotiationContentEntityTest.php

@@ -0,0 +1,196 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Core\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Session\Session;
+use Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Tests language negotiation with the language negotiator content entity.
+ *
+ * @group language
+ */
+class LanguageNegotiationContentEntityTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'language_test',
+    'entity_test',
+    'system',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * The entity being used for testing.
+   *
+   * @var \Drupal\Core\Entity\ContentEntityInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    ConfigurableLanguage::createFromLangcode('es')->save();
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+
+    // In order to reflect the changes for a multilingual site in the container
+    // we have to rebuild it.
+    $this->rebuildContainer();
+
+    $this->createTranslatableEntity();
+
+    $user = $this->drupalCreateUser(['view test entity']);
+    $this->drupalLogin($user);
+  }
+
+  /**
+   * Tests default with content language remaining same as interface language.
+   */
+  public function testDefaultConfiguration(): void {
+    $translation = $this->entity;
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    $this->assertSame($last_content_language, $last_interface_language);
+    $this->assertSame($translation->language()->getId(), $last_content_language);
+
+    $translation = $this->entity->getTranslation('es');
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    $this->assertSame($last_content_language, $last_interface_language);
+    $this->assertSame($translation->language()->getId(), $last_content_language);
+
+    $translation = $this->entity->getTranslation('fr');
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    $this->assertSame($last_content_language, $last_interface_language);
+    $this->assertSame($translation->language()->getId(), $last_content_language);
+  }
+
+  /**
+   * Tests enabling the language negotiator language_content_entity.
+   */
+  public function testEnabledLanguageContentNegotiator(): void {
+    // Define the method language-url with a higher priority than
+    // language-content-entity. This configuration should match the default one,
+    // where the language-content-entity is turned off.
+    $config = $this->config('language.types');
+    $config->set('configurable', [LanguageInterface::TYPE_INTERFACE, LanguageInterface::TYPE_CONTENT]);
+    $config->set('negotiation.language_content.enabled', [
+      LanguageNegotiationUrl::METHOD_ID => 0,
+      LanguageNegotiationContentEntity::METHOD_ID => 1,
+    ]);
+    $config->save();
+
+    // In order to reflect the changes for a multilingual site in the container
+    // we have to rebuild it.
+    $this->rebuildContainer();
+
+    // The tests for the default configuration should still pass.
+    $this->testDefaultConfiguration();
+
+    // Define the method language-content-entity with a higher priority than
+    // language-url.
+    $config->set('negotiation.language_content.enabled', [
+      LanguageNegotiationContentEntity::METHOD_ID => 0,
+      LanguageNegotiationUrl::METHOD_ID => 1,
+    ]);
+    $config->save();
+
+    // In order to reflect the changes for a multilingual site in the container
+    // we have to rebuild it.
+    $this->rebuildContainer();
+
+    // The method language-content-entity should run before language-url and
+    // append query parameter for the content language and prevent language-url
+    // from overwriting the URL.
+    $default_site_langcode = $this->config('system.site')->get('default_langcode');
+
+    // Now switching to an entity route, so that the URL links are generated
+    // while being on an entity route.
+    $this->setCurrentRequestForRoute('/entity_test/{entity_test}', 'entity.entity_test.canonical');
+
+    $translation = $this->entity;
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    // Check that interface language and content language are the same as the
+    // default translation language of the entity.
+    $this->assertSame($default_site_langcode, $last_interface_language);
+    $this->assertSame($last_content_language, $last_interface_language);
+    $this->assertSame($translation->language()->getId(), $last_content_language);
+
+    $translation = $this->entity->getTranslation('es');
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    $this->assertSame($last_interface_language, $default_site_langcode, 'Interface language did not change from the default site language.');
+    $this->assertSame($last_content_language, $translation->language()->getId(), 'Content language matches the current entity translation language.');
+
+    $translation = $this->entity->getTranslation('fr');
+    $this->drupalGet($translation->toUrl());
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    $last_content_language = $last[LanguageInterface::TYPE_CONTENT];
+    $last_interface_language = $last[LanguageInterface::TYPE_INTERFACE];
+    $this->assertSame($last_interface_language, $default_site_langcode, 'Interface language did not change from the default site language.');
+    $this->assertSame($last_content_language, $translation->language()->getId(), 'Content language matches the current entity translation language.');
+  }
+
+  /**
+   * Creates a translated entity.
+   */
+  protected function createTranslatableEntity() {
+    $this->entity = EntityTest::create();
+    $this->entity->addTranslation('es', ['name' => 'name spanish']);
+    $this->entity->addTranslation('fr', ['name' => 'name french']);
+    $this->entity->save();
+  }
+
+  /**
+   * Sets the current request to a specific path with the corresponding route.
+   *
+   * @param string $path
+   *   The path for which the current request should be created.
+   * @param string $route_name
+   *   The route name for which the route object for the request should be
+   *   created.
+   */
+  protected function setCurrentRequestForRoute($path, $route_name) {
+    $request = Request::create($path);
+    $request->attributes->set(RouteObjectInterface::ROUTE_NAME, $route_name);
+    $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route($path));
+    $request->setSession(new Session(new MockArraySessionStorage()));
+    $this->container->get('request_stack')->push($request);
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageNegotiationInfoTest.php

@@ -0,0 +1,223 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUI;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests alterations to language types/negotiation info.
+ *
+ * @group language
+ */
+class LanguageNegotiationInfoTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'content_translation'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'view the administration theme',
+      'administer modules',
+    ]);
+    $this->drupalLogin($admin_user);
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'it'], 'Add language');
+  }
+
+  /**
+   * Returns the configurable language manager.
+   *
+   * @return \Drupal\language\ConfigurableLanguageManager
+   */
+  protected function languageManager() {
+    return $this->container->get('language_manager');
+  }
+
+  /**
+   * Sets state flags for language_test module.
+   *
+   * Ensures to correctly update data both in the child site and the test runner
+   * environment.
+   *
+   * @param array $values
+   *   The key/value pairs to set in state.
+   */
+  protected function stateSet(array $values) {
+    // Set the new state values.
+    $this->container->get('state')->setMultiple($values);
+    // Refresh in-memory static state/config caches and static variables.
+    $this->refreshVariables();
+    // Refresh/rewrite language negotiation configuration, in order to pick up
+    // the manipulations performed by language_test module's info alter hooks.
+    $this->container->get('language_negotiator')->purgeConfiguration();
+  }
+
+  /**
+   * Tests alterations to language types/negotiation info.
+   */
+  public function testInfoAlterations(): void {
+    $this->stateSet([
+      // Enable language_test type info.
+      'language_test.language_types' => TRUE,
+      // Enable language_test negotiation info (not altered yet).
+      'language_test.language_negotiation_info' => TRUE,
+      // Alter LanguageInterface::TYPE_CONTENT to be configurable.
+      'language_test.content_language_type' => TRUE,
+    ]);
+    $this->container->get('module_installer')->install(['language_test']);
+    $this->resetAll();
+
+    // Check that fixed language types are properly configured without the need
+    // of saving the language negotiation settings.
+    $this->checkFixedLanguageTypes();
+
+    $type = LanguageInterface::TYPE_CONTENT;
+    $language_types = $this->languageManager()->getLanguageTypes();
+    $this->assertContains($type, $language_types, 'Content language type is configurable.');
+
+    // Enable some core and custom language negotiation methods. The test
+    // language type is supposed to be configurable.
+    $test_type = 'test_language_type';
+    $interface_method_id = LanguageNegotiationUI::METHOD_ID;
+    $test_method_id = 'test_language_negotiation_method';
+    $form_field = $type . '[enabled][' . $interface_method_id . ']';
+    $edit = [
+      $form_field => TRUE,
+      $type . '[enabled][' . $test_method_id . ']' => TRUE,
+      $test_type . '[enabled][' . $test_method_id . ']' => TRUE,
+      $test_type . '[configurable]' => TRUE,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Alter language negotiation info to remove interface language negotiation
+    // method.
+    $this->stateSet([
+      'language_test.language_negotiation_info_alter' => TRUE,
+    ]);
+
+    $negotiation = $this->config('language.types')->get('negotiation.' . $type . '.enabled');
+    $this->assertFalse(isset($negotiation[$interface_method_id]), 'Interface language negotiation method removed from the stored settings.');
+
+    // Check that the interface language negotiation method is unavailable.
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->assertSession()->fieldNotExists($form_field);
+
+    // Check that type-specific language negotiation methods can be assigned
+    // only to the corresponding language types.
+    foreach ($this->languageManager()->getLanguageTypes() as $type) {
+      $form_field = $type . '[enabled][test_language_negotiation_method_ts]';
+      if ($type == $test_type) {
+        $this->assertSession()->fieldExists($form_field);
+      }
+      else {
+        $this->assertSession()->fieldNotExists($form_field);
+      }
+    }
+
+    // Check language negotiation results.
+    $this->drupalGet('');
+    $last = \Drupal::keyValue('language_test')->get('language_negotiation_last');
+    foreach ($this->languageManager()->getDefinedLanguageTypes() as $type) {
+      $langcode = $last[$type];
+      $value = $type == LanguageInterface::TYPE_CONTENT || str_contains($type, 'test') ? 'it' : 'en';
+      $this->assertEquals($langcode, $value, "The negotiated language for $type is $value");
+    }
+
+    // Uninstall language_test and check that everything is set back to the
+    // original status.
+    $this->container->get('module_installer')->uninstall(['language_test']);
+    $this->rebuildContainer();
+
+    // Check that only the core language types are available.
+    foreach ($this->languageManager()->getDefinedLanguageTypes() as $type) {
+      $this->assertStringNotContainsString('test', $type, "The $type language is still available");
+    }
+
+    // Check that fixed language types are properly configured, even those
+    // previously set to configurable.
+    $this->checkFixedLanguageTypes();
+
+    // Check that unavailable language negotiation methods are not present in
+    // the negotiation settings.
+    $negotiation = $this->config('language.types')->get('negotiation.' . $type . '.enabled');
+    $this->assertFalse(isset($negotiation[$test_method_id]), 'The disabled test language negotiation method is not part of the content language negotiation settings.');
+
+    // Check that configuration page presents the correct options and settings.
+    $this->assertSession()->pageTextNotContains("Test language detection");
+    $this->assertSession()->pageTextNotContains("This is a test language negotiation method");
+  }
+
+  /**
+   * Check that language negotiation for fixed types matches the stored one.
+   */
+  protected function checkFixedLanguageTypes() {
+    $configurable = $this->languageManager()->getLanguageTypes();
+    foreach ($this->languageManager()->getDefinedLanguageTypesInfo() as $type => $info) {
+      if (!in_array($type, $configurable) && isset($info['fixed'])) {
+        $negotiation = $this->config('language.types')->get('negotiation.' . $type . '.enabled');
+        $equal = array_keys($negotiation) === array_values($info['fixed']);
+        $this->assertTrue($equal, "language negotiation for $type is properly set up");
+      }
+    }
+  }
+
+  /**
+   * Tests altering config of configurable language types.
+   */
+  public function testConfigLangTypeAlterations(): void {
+    // Default of config.
+    $test_type = LanguageInterface::TYPE_CONTENT;
+    $this->assertFalse($this->isLanguageTypeConfigurable($test_type), 'Language type is not configurable.');
+
+    // Editing config.
+    $edit = [$test_type . '[configurable]' => TRUE];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+    $this->assertTrue($this->isLanguageTypeConfigurable($test_type), 'Language type is now configurable.');
+
+    // After installing another module, the config should be the same.
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[test_module][enable]' => 1], 'Install');
+    $this->assertTrue($this->isLanguageTypeConfigurable($test_type), 'Language type is still configurable.');
+
+    // After uninstalling the other module, the config should be the same.
+    $this->drupalGet('admin/modules/uninstall');
+    $this->submitForm(['uninstall[test_module]' => 1], 'Uninstall');
+    $this->assertTrue($this->isLanguageTypeConfigurable($test_type), 'Language type is still configurable.');
+  }
+
+  /**
+   * Checks whether the given language type is configurable.
+   *
+   * @param string $type
+   *   The language type.
+   *
+   * @return bool
+   *   TRUE if the specified language type is configurable, FALSE otherwise.
+   */
+  protected function isLanguageTypeConfigurable($type) {
+    $configurable_types = $this->config('language.types')->get('configurable');
+    return in_array($type, $configurable_types);
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageNegotiationSessionTest.php

@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the session language negotiation method.
+ *
+ * @group language
+ */
+class LanguageNegotiationSessionTest extends BrowserTestBase {
+
+  /**
+   * An administrative user to configure the test environment.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $adminUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create a new user with permission to manage the languages.
+    $this->adminUser = $this->drupalCreateUser(['administer languages']);
+    $this->drupalLogin($this->adminUser);
+
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+  }
+
+  /**
+   * Tests language negotiation via query/session parameters.
+   */
+  public function testSessionLanguageNegotiationMethod(): void {
+    // Enable Session and Selected language for interface language detection.
+    $this->drupalGet('admin/config/regional/language/detection');
+    $edit = [
+      'language_interface[enabled][language-session]' => 1,
+      'language_interface[enabled][language-selected]' => 1,
+      'language_interface[weight][language-session]' => -6,
+      'language_interface[weight][language-selected]' => 12,
+    ];
+    $this->submitForm($edit, 'Save settings');
+
+    // Set language via query parameter.
+    $this->drupalGet('user/' . $this->adminUser->id(), ['query' => ['language' => 'fr']]);
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->responseHeaderEquals('Content-language', 'fr');
+
+    // Verify that the language is persisted in the session.
+    $this->drupalGet('user/' . $this->adminUser->id());
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->responseHeaderEquals('Content-language', 'fr');
+
+    // Set language via query parameter.
+    $this->drupalGet('user/' . $this->adminUser->id(), ['query' => ['language' => 'en']]);
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->responseHeaderEquals('Content-language', 'en');
+
+    // Verify that the language is persisted in the session.
+    $this->drupalGet('admin/config/regional/language');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->responseHeaderEquals('Content-language', 'en');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageNegotiationUrlTest.php

@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl
+ * @group language
+ */
+class LanguageNegotiationUrlTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'node',
+    'path',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * @var \Drupal\user\Entity\User
+   */
+  protected $user;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create an Article node type.
+    if ($this->profile != 'standard') {
+      $this->drupalCreateContentType(['type' => 'article']);
+    }
+
+    $this->user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'view the administration theme',
+      'administer nodes',
+      'create article content',
+      'create url aliases',
+    ]);
+    $this->drupalLogin($this->user);
+
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'de'], 'Add language');
+  }
+
+  /**
+   * @covers ::processInbound
+   */
+  public function testDomain(): void {
+    // Check if paths that contain language prefixes can be reached when
+    // language is taken from the domain.
+    $edit = [
+      'language_negotiation_url_part' => 'domain',
+      'prefix[en]' => 'eng',
+      'prefix[de]' => 'de',
+      'domain[en]' => $_SERVER['HTTP_HOST'],
+      'domain[de]' => "de.$_SERVER[HTTP_HOST]",
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+
+    $nodeValues = [
+      'title[0][value]' => 'Test',
+      'path[0][alias]' => '/eng/test',
+    ];
+    $this->drupalGet('node/add/article');
+    $this->submitForm($nodeValues, 'Save');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
+}

core/modules/language/tests/src/Functional/LanguagePathMonolingualTest.php

@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Confirm that paths are not changed on monolingual non-English sites.
+ *
+ * @group language
+ */
+class LanguagePathMonolingualTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['block', 'language', 'path'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create and log in user.
+    $web_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'administer site configuration',
+    ]);
+    $this->drupalLogin($web_user);
+
+    // Enable French language.
+    $edit = [];
+    $edit['predefined_langcode'] = 'fr';
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Make French the default language.
+    $edit = [
+      'site_default_language' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Delete English.
+    $this->drupalGet('admin/config/regional/language/delete/en');
+    $this->submitForm([], 'Delete');
+
+    // Changing the default language causes a container rebuild. Therefore need
+    // to rebuild the container in the test environment.
+    $this->rebuildContainer();
+
+    // Verify that French is the only language.
+    $this->container->get('language_manager')->reset();
+    $this->assertFalse(\Drupal::languageManager()->isMultilingual(), 'Site is mono-lingual');
+    $this->assertEquals('fr', \Drupal::languageManager()->getDefaultLanguage()->getId(), 'French is the default language');
+
+    // Set language detection to URL.
+    $edit = ['language_interface[enabled][language-url]' => TRUE];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+    $this->drupalPlaceBlock('local_actions_block');
+  }
+
+  /**
+   * Verifies that links do not have language prefixes in them.
+   */
+  public function testPageLinks(): void {
+    // Navigate to 'admin/config' path.
+    $this->drupalGet('admin/config');
+
+    // Verify that links in this page do not have a 'fr/' prefix.
+    $this->assertSession()->linkByHrefNotExists('/fr/', 'Links do not contain language prefix');
+
+    // Verify that links in this page can be followed and work.
+    $this->clickLink('Languages');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->pageTextContains('Add language');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageSelectorTranslatableTest.php

@@ -0,0 +1,112 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\language\Traits\LanguageTestTrait;
+
+/**
+ * Tests the content translation settings language selector options.
+ *
+ * @covers \Drupal\language\Form\ContentLanguageSettingsForm
+ * @group language
+ */
+class LanguageSelectorTranslatableTest extends BrowserTestBase {
+
+  use LanguageTestTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'content_translation',
+    'node',
+    'comment',
+    'field_ui',
+    'entity_test',
+    'locale',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * The user with administrator privileges.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  public $administrator;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create user and set permissions.
+    $this->administrator = $this->drupalCreateUser($this->getAdministratorPermissions(), 'administrator');
+    $this->drupalLogin($this->administrator);
+  }
+
+  /**
+   * Returns an array of permissions needed for the translator.
+   */
+  protected function getAdministratorPermissions() {
+    return array_filter(
+      ['translate interface',
+        'administer content translation',
+        'create content translations',
+        'update content translations',
+        'delete content translations',
+        'administer languages',
+      ]
+    );
+  }
+
+  /**
+   * Tests content translation language selectors are correctly translated.
+   */
+  public function testLanguageStringSelector(): void {
+    // Add another language.
+    static::createLanguageFromLangcode('es');
+
+    // Translate the string English in Spanish (Inglés). Override config entity.
+    $name_translation = 'Inglés';
+    \Drupal::languageManager()
+      ->getLanguageConfigOverride('es', 'language.entity.en')
+      ->set('label', $name_translation)
+      ->save();
+
+    // Check content translation overview selector.
+    $path = 'es/admin/config/regional/content-language';
+    $this->drupalGet($path);
+
+    // Get en language from selector.
+    $option = $this->assertSession()->optionExists('edit-settings-user-user-settings-language-langcode', 'en');
+
+    // Check that the language text is translated.
+    $this->assertSame($name_translation, $option->getText());
+  }
+
+  /**
+   * Tests that correct title is displayed for content translation page.
+   */
+  public function testContentTranslationPageTitle(): void {
+    $this->drupalGet('admin/config/regional/content-language');
+    $this->assertSession()->pageTextContains('Content language and translation');
+    $this->assertSession()->pageTextNotMatches('#Content language$#');
+
+    \Drupal::service('module_installer')->uninstall(['content_translation']);
+    $this->drupalGet('admin/config/regional/content-language');
+    $this->assertSession()->pageTextContains('Content language');
+    $this->assertSession()->pageTextNotContains('Content language and translation');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageSwitchingTest.php

@@ -0,0 +1,732 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Core\Url;
+
+// cspell:ignore publi publié
+
+/**
+ * Functional tests for the language switching feature.
+ *
+ * @group language
+ */
+class LanguageSwitchingTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'locale',
+    'locale_test',
+    'language',
+    'block',
+    'language_test',
+    'menu_ui',
+    'node',
+  ];
+
+  /**
+   * The theme to install as the default for testing.
+   *
+   * @var string
+   */
+  protected $defaultTheme = 'starterkit_theme';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create and log in user.
+    $admin_user = $this->drupalCreateUser([
+      'administer blocks',
+      'administer languages',
+      'administer site configuration',
+      'access administration pages',
+      'access content',
+    ]);
+    $this->drupalLogin($admin_user);
+  }
+
+  /**
+   * Functional tests for the language switcher block.
+   */
+  public function testLanguageBlock(): void {
+    // Add language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Set the native language name.
+    $this->saveNativeLanguageName('fr', 'français');
+
+    // Enable URL language detection and selection.
+    $edit = ['language_interface[enabled][language-url]' => '1'];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Enable the language switching block.
+    $block = $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_INTERFACE, [
+      'id' => 'test_language_block',
+      // Ensure a 2-byte UTF-8 sequence is in the tested output.
+      'label' => $this->randomMachineName(8) . '×',
+    ]);
+
+    $this->doTestLanguageBlockAuthenticated($block->label());
+    $this->doTestHomePageLinks($block->label());
+    $this->doTestLanguageBlockAnonymous($block->label());
+    $this->doTestLanguageBlock404($block->label(), 'system/404');
+
+    // Test 404s with big_pipe where the behavior is different for logged-in
+    // users.
+    \Drupal::service('module_installer')->install(['big_pipe']);
+    $this->rebuildAll();
+    $this->doTestLanguageBlock404($block->label(), 'system/404');
+    $this->drupalLogin($this->drupalCreateUser());
+    // @todo This is testing the current behavior with the big_pipe module
+    //   enabled. This behavior is a bug will be fixed in
+    //   https://www.drupal.org/project/drupal/issues/3349201.
+    $this->doTestLanguageBlock404($block->label(), '<front>');
+  }
+
+  /**
+   * The home page link should be "/" or "/{language_prefix}".
+   *
+   * @param string $block_label
+   *   The label of the language switching block.
+   *
+   * @see self::testLanguageBlock()
+   */
+  protected function doTestHomePageLinks($block_label) {
+    // Create a node and set as home page.
+    $this->createHomePage();
+    // Go to home page.
+    $this->DrupalGet('<front>');
+    // The language switcher block should display.
+    $this->assertSession()->pageTextContains($block_label);
+    // Assert that each list item and anchor element has the appropriate data-
+    // attributes.
+    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', [':id' => 'block-test-language-block']);
+    $list_items = [];
+    $anchors = [];
+    $labels = [];
+    foreach ($language_switchers as $list_item) {
+      $list_items[] = [
+        'hreflang' => $list_item->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $list_item->getAttribute('data-drupal-link-system-path'),
+      ];
+
+      $link = $list_item->find('xpath', 'a');
+      $anchors[] = [
+        'hreflang' => $link->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $link->getAttribute('data-drupal-link-system-path'),
+        'href' => $link->getAttribute('href'),
+      ];
+      $labels[] = $link->getText();
+    }
+    $expected_list_items = [
+      0 => [
+        'hreflang' => 'en',
+        'data-drupal-link-system-path' => '<front>',
+      ],
+      1 => [
+        'hreflang' => 'fr',
+        'data-drupal-link-system-path' => '<front>',
+      ],
+    ];
+    $this->assertSame($expected_list_items, $list_items, 'The list items have the correct attributes that will contain the correct home page links.');
+    $expected_anchors = [
+      0 => [
+        'hreflang' => 'en',
+        'data-drupal-link-system-path' => '<front>',
+        'href' => Url::fromRoute('<front>')->toString(),
+      ],
+      1 => [
+        'hreflang' => 'fr',
+        'data-drupal-link-system-path' => '<front>',
+        'href' => Url::fromRoute('<front>')->toString() . 'fr',
+      ],
+    ];
+    $this->assertSame($expected_anchors, $anchors, 'The anchors have the correct attributes that will link to the correct home page in that language.');
+    $this->assertSame(['English', 'français'], $labels, 'The language links labels are in their own language on the language switcher block.');
+  }
+
+  /**
+   * For authenticated users, the "active" class is set by JavaScript.
+   *
+   * @param string $block_label
+   *   The label of the language switching block.
+   *
+   * @see self::testLanguageBlock()
+   */
+  protected function doTestLanguageBlockAuthenticated($block_label) {
+    // Assert that the language switching block is displayed on the frontpage.
+    $this->drupalGet('');
+    $this->assertSession()->pageTextContains($block_label);
+
+    // Assert that each list item and anchor element has the appropriate data-
+    // attributes.
+    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', [':id' => 'block-test-language-block']);
+    $list_items = [];
+    $anchors = [];
+    $labels = [];
+    foreach ($language_switchers as $list_item) {
+      $list_items[] = [
+        'hreflang' => $list_item->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $list_item->getAttribute('data-drupal-link-system-path'),
+      ];
+
+      $link = $list_item->find('xpath', 'a');
+      $anchors[] = [
+        'hreflang' => $link->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $link->getAttribute('data-drupal-link-system-path'),
+      ];
+      $labels[] = $link->getText();
+    }
+    $expected_list_items = [
+      0 => ['hreflang' => 'en', 'data-drupal-link-system-path' => 'user/2'],
+      1 => ['hreflang' => 'fr', 'data-drupal-link-system-path' => 'user/2'],
+    ];
+    $this->assertSame($expected_list_items, $list_items, 'The list items have the correct attributes that will allow the drupal.active-link library to mark them as active.');
+    $expected_anchors = [
+      0 => ['hreflang' => 'en', 'data-drupal-link-system-path' => 'user/2'],
+      1 => ['hreflang' => 'fr', 'data-drupal-link-system-path' => 'user/2'],
+    ];
+    $this->assertSame($expected_anchors, $anchors, 'The anchors have the correct attributes that will allow the drupal.active-link library to mark them as active.');
+    $settings = $this->getDrupalSettings();
+    $this->assertSame('user/2', $settings['path']['currentPath'], 'drupalSettings.path.currentPath is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertFalse($settings['path']['isFront'], 'drupalSettings.path.isFront is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertSame('en', $settings['path']['currentLanguage'], 'drupalSettings.path.currentLanguage is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertSame(['English', 'français'], $labels, 'The language links labels are in their own language on the language switcher block.');
+  }
+
+  /**
+   * For anonymous users, the "active" class is set by PHP.
+   *
+   * @param string $block_label
+   *   The label of the language switching block.
+   *
+   * @see self::testLanguageBlock()
+   */
+  protected function doTestLanguageBlockAnonymous($block_label) {
+    $this->drupalLogout();
+
+    // Assert that the language switching block is displayed on the frontpage
+    // and ensure that the active class is added when query params are present.
+    $this->drupalGet('', ['query' => ['foo' => 'bar']]);
+    $this->assertSession()->pageTextContains($block_label);
+
+    // Assert that only the current language is marked as active.
+    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', [':id' => 'block-test-language-block']);
+    $links = [
+      'active' => [],
+      'inactive' => [],
+    ];
+    $anchors = [
+      'active' => [],
+      'inactive' => [],
+    ];
+    $labels = [];
+    foreach ($language_switchers as $list_item) {
+      $langcode = $list_item->getAttribute('hreflang');
+      if ($list_item->hasClass('is-active')) {
+        $links['active'][] = $langcode;
+      }
+      else {
+        $links['inactive'][] = $langcode;
+      }
+
+      $link = $list_item->find('xpath', 'a');
+      $anchor_classes = explode(" ", $link->getAttribute('class'));
+      if (in_array('is-active', $anchor_classes)) {
+        $anchors['active'][] = $langcode;
+      }
+      else {
+        $anchors['inactive'][] = $langcode;
+      }
+      $labels[] = $link->getText();
+    }
+    $this->assertSame(['active' => ['en'], 'inactive' => ['fr']], $links, 'Only the current language list item is marked as active on the language switcher block.');
+    $this->assertSame(['active' => ['en'], 'inactive' => ['fr']], $anchors, 'Only the current language anchor is marked as active on the language switcher block.');
+    $this->assertSame(['English', 'français'], $labels, 'The language links labels are in their own language on the language switcher block.');
+  }
+
+  /**
+   * Tests the language switcher block on 404 pages.
+   *
+   * @param string $block_label
+   *   The label of the language switching block.
+   * @param string $system_path
+   *   The expected system path for the links in the language switcher.
+   *
+   * @see self::testLanguageBlock()
+   */
+  protected function doTestLanguageBlock404(string $block_label, string $system_path) {
+    $this->drupalGet('does-not-exist-' . $this->randomMachineName());
+    $this->assertSession()->pageTextContains($block_label);
+
+    // Assert that each list item and anchor element has the appropriate data-
+    // attributes.
+    $language_switchers = $this->xpath('//div[@id=:id]/ul/li', [':id' => 'block-test-language-block']);
+    $list_items = [];
+    $anchors = [];
+    $labels = [];
+    foreach ($language_switchers as $list_item) {
+      $list_items[] = [
+        'hreflang' => $list_item->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $list_item->getAttribute('data-drupal-link-system-path'),
+      ];
+
+      $link = $list_item->find('xpath', 'a');
+      $anchors[] = [
+        'hreflang' => $link->getAttribute('hreflang'),
+        'data-drupal-link-system-path' => $link->getAttribute('data-drupal-link-system-path'),
+      ];
+      $labels[] = $link->getText();
+    }
+    $expected_list_items = [
+      0 => ['hreflang' => 'en', 'data-drupal-link-system-path' => $system_path],
+      1 => ['hreflang' => 'fr', 'data-drupal-link-system-path' => $system_path],
+    ];
+    $this->assertSame($expected_list_items, $list_items, 'The list items have the correct attributes that will allow the drupal.active-link library to mark them as active.');
+    $expected_anchors = [
+      0 => ['hreflang' => 'en', 'data-drupal-link-system-path' => $system_path],
+      1 => ['hreflang' => 'fr', 'data-drupal-link-system-path' => $system_path],
+    ];
+    $this->assertSame($expected_anchors, $anchors, 'The anchors have the correct attributes that will allow the drupal.active-link library to mark them as active.');
+    $this->assertSame(['English', 'français'], $labels, 'The language links labels are in their own language on the language switcher block.');
+  }
+
+  /**
+   * Tests language switcher links for domain based negotiation.
+   */
+  public function testLanguageBlockWithDomain(): void {
+    // Add the Italian language.
+    ConfigurableLanguage::createFromLangcode('it')->save();
+
+    // Rebuild the container so that the new language is picked up by services
+    // that hold a list of languages.
+    $this->rebuildContainer();
+
+    $languages = $this->container->get('language_manager')->getLanguages();
+
+    // Enable browser and URL language detection.
+    $edit = [
+      'language_interface[enabled][language-url]' => TRUE,
+      'language_interface[weight][language-url]' => -10,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Do not allow blank domain.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => '',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The domain may not be left blank for English', 'error');
+
+    // Change the domain for the Italian language.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => \Drupal::request()->getHost(),
+      'domain[it]' => 'it.example.com',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The configuration options have been saved', 'status');
+
+    // Enable the language switcher block.
+    $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_INTERFACE, ['id' => 'test_language_block']);
+
+    $this->drupalGet('');
+
+    /** @var \Drupal\Core\Routing\UrlGenerator $generator */
+    $generator = $this->container->get('url_generator');
+
+    // Verify the English URL is correct
+    $english_url = $generator->generateFromRoute('entity.user.canonical', ['user' => 2], ['language' => $languages['en']]);
+    $this->assertSession()->elementAttributeContains('xpath', '//div[@id="block-test-language-block"]/ul/li/a[@hreflang="en"]', 'href', $english_url);
+
+    // Verify the Italian URL is correct
+    $italian_url = $generator->generateFromRoute('entity.user.canonical', ['user' => 2], ['language' => $languages['it']]);
+    $this->assertSession()->elementAttributeContains('xpath', '//div[@id="block-test-language-block"]/ul/li/a[@hreflang="it"]', 'href', $italian_url);
+  }
+
+  /**
+   * Tests active class on links when switching languages.
+   */
+  public function testLanguageLinkActiveClass(): void {
+    // Add language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Enable URL language detection and selection.
+    $edit = ['language_interface[enabled][language-url]' => '1'];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    $this->doTestLanguageLinkActiveClassAuthenticated();
+    $this->doTestLanguageLinkActiveClassAnonymous();
+  }
+
+  /**
+   * Check the path-admin class, as same as on default language.
+   */
+  public function testLanguageBodyClass(): void {
+    // Add language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Enable URL language detection and selection.
+    $edit = ['language_interface[enabled][language-url]' => '1'];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Check if the default (English) admin/config page has the right class.
+    $this->drupalGet('admin/config');
+    $this->assertSession()->elementAttributeContains('xpath', '//body', 'class', 'path-admin');
+
+    // Check if the French admin/config page has the right class.
+    $this->drupalGet('fr/admin/config');
+    $this->assertSession()->elementAttributeContains('xpath', '//body', 'class', 'path-admin');
+
+    // The testing profile sets the user/login page as the frontpage. That
+    // redirects authenticated users to their profile page, so check with an
+    // anonymous user instead.
+    $this->drupalLogout();
+
+    // Check if the default (English) frontpage has the right class.
+    $this->drupalGet('<front>');
+    $this->assertSession()->elementAttributeContains('xpath', '//body', 'class', 'path-frontpage');
+
+    // Check if the French frontpage has the right class.
+    $this->drupalGet('fr');
+    $this->assertSession()->elementAttributeContains('xpath', '//body', 'class', 'path-frontpage');
+  }
+
+  /**
+   * For authenticated users, the "active" class is set by JavaScript.
+   *
+   * @see self::testLanguageLinkActiveClass()
+   */
+  protected function doTestLanguageLinkActiveClassAuthenticated() {
+    $function_name = '#type link';
+    $path = 'language_test/type-link-active-class';
+
+    // Test links generated by the link generator on an English page.
+    $current_language = 'English';
+    $this->drupalGet($path);
+
+    // Language code 'none' link should be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'no_lang_link'], 'data-drupal-link-system-path', $path);
+
+    // Language code 'en' link should be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'en_link'], 'hreflang', 'en');
+    $this->assertSession()->elementAttributeContains('named', ['id', 'en_link'], 'data-drupal-link-system-path', $path);
+
+    // Language code 'fr' link should not be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'fr_link'], 'hreflang', 'fr');
+    $this->assertSession()->elementAttributeContains('named', ['id', 'fr_link'], 'data-drupal-link-system-path', $path);
+
+    // Verify that drupalSettings contains the correct values.
+    $settings = $this->getDrupalSettings();
+    $this->assertSame($path, $settings['path']['currentPath'], 'drupalSettings.path.currentPath is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertFalse($settings['path']['isFront'], 'drupalSettings.path.isFront is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertSame('en', $settings['path']['currentLanguage'], 'drupalSettings.path.currentLanguage is set correctly to allow drupal.active-link to mark the correct links as active.');
+
+    // Test links generated by the link generator on a French page.
+    $current_language = 'French';
+    $this->drupalGet('fr/language_test/type-link-active-class');
+
+    // Language code 'none' link should be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'no_lang_link'], 'data-drupal-link-system-path', $path);
+
+    // Language code 'en' link should not be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'en_link'], 'hreflang', 'en');
+    $this->assertSession()->elementAttributeContains('named', ['id', 'en_link'], 'data-drupal-link-system-path', $path);
+
+    // Language code 'fr' link should be active.
+    $this->assertSession()->elementAttributeContains('named', ['id', 'fr_link'], 'hreflang', 'fr');
+    $this->assertSession()->elementAttributeContains('named', ['id', 'fr_link'], 'data-drupal-link-system-path', $path);
+
+    // Verify that drupalSettings contains the correct values.
+    $settings = $this->getDrupalSettings();
+    $this->assertSame($path, $settings['path']['currentPath'], 'drupalSettings.path.currentPath is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertFalse($settings['path']['isFront'], 'drupalSettings.path.isFront is set correctly to allow drupal.active-link to mark the correct links as active.');
+    $this->assertSame('fr', $settings['path']['currentLanguage'], 'drupalSettings.path.currentLanguage is set correctly to allow drupal.active-link to mark the correct links as active.');
+  }
+
+  /**
+   * For anonymous users, the "active" class is set by PHP.
+   *
+   * @see self::testLanguageLinkActiveClass()
+   */
+  protected function doTestLanguageLinkActiveClassAnonymous() {
+    $function_name = '#type link';
+
+    $this->drupalLogout();
+
+    // Test links generated by the link generator on an English page.
+    $current_language = 'English';
+    $this->drupalGet('language_test/type-link-active-class');
+
+    // Language code 'none' link should be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'no_lang_link' and contains(@class, 'is-active')]");
+
+    // Language code 'en' link should be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'en_link' and contains(@class, 'is-active')]");
+
+    // Language code 'fr' link should not be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'fr_link' and not(contains(@class, 'is-active'))]");
+
+    // Test links generated by the link generator on a French page.
+    $current_language = 'French';
+    $this->drupalGet('fr/language_test/type-link-active-class');
+
+    // Language code 'none' link should be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'no_lang_link' and contains(@class, 'is-active')]");
+
+    // Language code 'en' link should not be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'en_link' and not(contains(@class, 'is-active'))]");
+
+    // Language code 'fr' link should be active.
+    $this->assertSession()->elementExists('xpath', "//a[@id = 'fr_link' and contains(@class, 'is-active')]");
+  }
+
+  /**
+   * Tests language switcher links for session based negotiation.
+   */
+  public function testLanguageSessionSwitchLinks(): void {
+    // Add language.
+    $edit = [
+      'predefined_langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Enable session language detection and selection.
+    $edit = [
+      'language_interface[enabled][language-url]' => FALSE,
+      'language_interface[enabled][language-session]' => TRUE,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Enable the language switching block.
+    $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_INTERFACE, [
+      'id' => 'test_language_block',
+    ]);
+
+    // Enable the main menu block.
+    $this->drupalPlaceBlock('system_menu_block:main', [
+      'id' => 'test_menu',
+    ]);
+
+    // Add a link to the homepage.
+    $link = MenuLinkContent::create([
+      'title' => 'Home',
+      'menu_name' => 'main',
+      'bundle' => 'menu_link_content',
+      'link' => [['uri' => 'entity:user/2']],
+    ]);
+    $link->save();
+
+    // Go to the homepage.
+    $this->drupalGet('');
+    // Click on the French link.
+    $this->clickLink('French');
+    // There should be a query parameter to set the session language.
+    $this->assertSession()->addressEquals('user/2?language=fr');
+    // Click on the 'Home' Link.
+    $this->clickLink('Home');
+    // There should be no query parameter.
+    $this->assertSession()->addressEquals('user/2');
+    // Click on the French link.
+    $this->clickLink('French');
+    // There should be no query parameter.
+    $this->assertSession()->addressEquals('user/2');
+  }
+
+  /**
+   * Test that the language switching block does not expose restricted paths.
+   */
+  public function testRestrictedPaths(): void {
+    $entity_type_manager = \Drupal::entityTypeManager();
+
+    // Add the French language.
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+
+    // Enable URL language detection and selection.
+    $this->config('language.types')
+      ->set('negotiation.language_interface.enabled.language-url', 1)
+      ->save();
+
+    // Enable the language switching block.
+    $block = $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_INTERFACE);
+
+    // Create a node type and make it translatable.
+    $entity_type_manager->getStorage('node_type')
+      ->create([
+        'type' => 'page',
+        'name' => 'Page',
+      ])
+      ->save();
+
+    // Create a published node with an unpublished translation.
+    $node = $entity_type_manager->getStorage('node')
+      ->create([
+        'type' => 'page',
+        'title' => $this->randomMachineName(),
+        'status' => 1,
+      ]);
+    $node->save();
+    $node->addTranslation('fr', ['title' => 'Non publié', 'status' => 0]);
+    $node->save();
+
+    // Create path aliases.
+    $alias_storage = $entity_type_manager->getStorage('path_alias');
+    $alias_storage->create([
+      'path' => '/user/1',
+      'alias' => '/secret-identity/peter-parker',
+    ])->save();
+    $alias_storage->create([
+      'path' => '/node/1',
+      'langcode' => 'en',
+      'alias' => '/press-release/published-report',
+    ])->save();
+    $alias_storage->create([
+      'path' => '/node/1',
+      'langcode' => 'fr',
+      'alias' => '/press-release/rapport-non-publié',
+    ])->save();
+
+    // Visit a restricted user page.
+    // Assert that the language switching block is displayed on the
+    // access-denied page, but it does not contain the path alias.
+    $this->assertLinkMarkup('/user/1', 403, $block->label(), 'peter-parker');
+
+    // Visit the node and its translation. Use internal paths and aliases. The
+    // non-ASCII character may be escaped, so remove it from the search string.
+    $this->assertLinkMarkup('/node/1', 200, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/press-release/published-report', 200, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/fr/node/1', 403, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/fr/press-release/rapport-non-publié', 403, $block->label(), 'rapport-non-publi');
+
+    // Test as a user with access to other users and unpublished content.
+    $privileged_user = $this->drupalCreateUser([
+      'access user profiles',
+      'bypass node access',
+    ]);
+    $this->drupalLogin($privileged_user);
+    $this->assertLinkMarkup('/user/1', 200, $block->label(), 'peter-parker', TRUE);
+    $this->assertLinkMarkup('/node/1', 200, $block->label(), 'rapport-non-publi', TRUE);
+    $this->assertLinkMarkup('/press-release/published-report', 200, $block->label(), 'rapport-non-publi', TRUE);
+    $this->assertLinkMarkup('/fr/node/1', 200, $block->label(), 'rapport-non-publi', TRUE);
+    $this->assertLinkMarkup('/fr/press-release/rapport-non-publié', 200, $block->label(), 'rapport-non-publi', TRUE);
+
+    // Test as an anonymous user.
+    $this->drupalLogout();
+    $this->assertLinkMarkup('/user/1', 403, $block->label(), 'peter-parker');
+    $this->assertLinkMarkup('/node/1', 200, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/press-release/published-report', 200, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/fr/node/1', 403, $block->label(), 'rapport-non-publi');
+    $this->assertLinkMarkup('/fr/press-release/rapport-non-publié', 403, $block->label(), 'rapport-non-publi');
+  }
+
+  /**
+   * Asserts that restricted text is or is not present in the page response.
+   *
+   * @param string $path
+   *   The path to test.
+   * @param int $status
+   *   The HTTP status code, such as 200 or 403.
+   * @param string $marker
+   *   Text that should always be present.
+   * @param string $restricted
+   *   Text that should be tested.
+   * @param bool $found
+   *   (optional) If TRUE, then the restricted text is present. Defaults to
+   *   FALSE.
+   */
+  protected function assertLinkMarkup(string $path, int $status, string $marker, string $restricted, bool $found = FALSE): void {
+    $this->drupalGet($path);
+    $this->assertSession()->statusCodeEquals($status);
+    $this->assertSession()->pageTextContains($marker);
+    if ($found) {
+      $this->assertSession()->responseContains($restricted);
+    }
+    else {
+      $this->assertSession()->responseNotContains($restricted);
+    }
+
+    // Assert that all languages had a link passed to
+    // hook_language_switch_links_alter() to allow alternatives to be provided.
+    $languages = \Drupal::languageManager()->getNativeLanguages();
+    $links_for_alter = \Drupal::state()->get('language_test.language_switch_link_ids');
+    $this->assertSame(array_keys($languages), $links_for_alter);
+  }
+
+  /**
+   * Saves the native name of a language entity in configuration as a label.
+   *
+   * @param string $langcode
+   *   The language code of the language.
+   * @param string $label
+   *   The native name of the language.
+   */
+  protected function saveNativeLanguageName($langcode, $label) {
+    \Drupal::service('language.config_factory_override')
+      ->getOverride($langcode, 'language.entity.' . $langcode)->set('label', $label)->save();
+  }
+
+  /**
+   * Create a node and set it as the home pages.
+   */
+  protected function createHomePage() {
+    $entity_type_manager = \Drupal::entityTypeManager();
+
+    // Create a node type and make it translatable.
+    $entity_type_manager->getStorage('node_type')
+      ->create([
+        'type' => 'page',
+        'name' => 'Page',
+      ])
+      ->save();
+
+    // Create a published node.
+    $node = $entity_type_manager->getStorage('node')
+      ->create([
+        'type' => 'page',
+        'title' => $this->randomMachineName(),
+        'status' => 1,
+      ]);
+    $node->save();
+
+    // Change the front page to /node/1.
+    $edit = ['site_frontpage' => '/node/1'];
+    $this->drupalGet('admin/config/system/site-information');
+    $this->submitForm($edit, 'Save configuration');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php

@@ -0,0 +1,626 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Url;
+use Drupal\file\Entity\File;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationBrowser;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationSelected;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationSession;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\user\Plugin\LanguageNegotiation\LanguageNegotiationUser;
+use Drupal\user\Plugin\LanguageNegotiation\LanguageNegotiationUserAdmin;
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Drupal\language\LanguageNegotiatorInterface;
+use Drupal\block\Entity\Block;
+use Symfony\Component\HttpFoundation\Session\Session;
+use Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage;
+
+/**
+ * Tests the language UI for language switching.
+ *
+ * The uses cases that get tested, are:
+ * - URL (path) > default: Test that the URL prefix setting gets precedence over
+ *   the default language. The browser language preference does not have any
+ *   influence.
+ * - URL (path) > browser > default: Test that the URL prefix setting gets
+ *   precedence over the browser language preference, which in turn gets
+ *   precedence over the default language.
+ * - URL (domain) > default: Tests that the URL domain setting gets precedence
+ *   over the default language.
+ *
+ * The paths that are used for each of these, are:
+ * - admin/config: Tests the UI using the precedence rules.
+ * - zh-hans/admin/config: Tests the UI in Chinese.
+ * - blah-blah/admin/config: Tests the 404 page.
+ *
+ * @group language
+ */
+class LanguageUILanguageNegotiationTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $configSchemaCheckerExclusions = [
+    // Necessary to allow setting `selected_langcode` to NULL.
+    // @see testUILanguageNegotiation()
+    'language.negotiation',
+  ];
+
+  /**
+   * The admin user for testing.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $adminUser;
+
+  /**
+   * Modules to enable.
+   *
+   * We marginally use interface translation functionality here, so need to use
+   * the locale module instead of language only, but the 90% of the test is
+   * about the negotiation process which is solely in language module.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'locale',
+    'language_test',
+    'block',
+    'user',
+    'content_translation',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->adminUser = $this->drupalCreateUser([
+      'administer languages',
+      'translate interface',
+      'access administration pages',
+      'administer blocks',
+    ]);
+    $this->drupalLogin($this->adminUser);
+  }
+
+  /**
+   * Tests for language switching by URL path.
+   */
+  public function testUILanguageNegotiation(): void {
+    // A few languages to switch to.
+    // This one is unknown, should get the default lang version.
+    $langcode_unknown = 'blah-blah';
+    // For testing browser lang preference.
+    $langcode_browser_fallback = 'vi';
+    // For testing path prefix.
+    $langcode = 'zh-hans';
+    // For setting browser language preference to 'vi'.
+    $http_header_browser_fallback = ["Accept-Language" => "$langcode_browser_fallback;q=1"];
+    // For setting browser language preference to some unknown.
+    $http_header_blah = ["Accept-Language" => "blah;q=1"];
+
+    // Create a private file for testing accessible by the admin user.
+    \Drupal::service('file_system')->mkdir($this->privateFilesDirectory . '/test');
+    $filepath = 'private://test/private-file-test.txt';
+    $contents = "file_put_contents() doesn't seem to appreciate empty strings so let's put in some data.";
+    file_put_contents($filepath, $contents);
+    $file = File::create([
+      'uri' => $filepath,
+      'uid' => $this->adminUser->id(),
+    ]);
+    $file->save();
+
+    // Setup the site languages by installing two languages.
+    // Set the default language in order for the translated string to be registered
+    // into database when seen by t(). Without doing this, our target string
+    // is for some reason not found when doing translate search. This might
+    // be some bug.
+    $default_language = \Drupal::languageManager()->getDefaultLanguage();
+    ConfigurableLanguage::createFromLangcode($langcode_browser_fallback)->save();
+    $this->config('system.site')->set('default_langcode', $langcode_browser_fallback)->save();
+    ConfigurableLanguage::createFromLangcode($langcode)->save();
+
+    // We will look for this string in the admin/config screen to see if the
+    // corresponding translated string is shown.
+    $default_string = 'Hide descriptions';
+
+    // First visit this page to make sure our target string is searchable.
+    $this->drupalGet('admin/config');
+
+    // Now the t()'ed string is in db so switch the language back to default.
+    // This will rebuild the container so we need to rebuild the container in
+    // the test environment.
+    $this->config('system.site')->set('default_langcode', $default_language->getId())->save();
+    $this->config('language.negotiation')->set('url.prefixes.en', '')->save();
+    $this->rebuildContainer();
+
+    // Translate the string.
+    $language_browser_fallback_string = "In $langcode_browser_fallback In $langcode_browser_fallback In $langcode_browser_fallback";
+    $language_string = "In $langcode In $langcode In $langcode";
+    // Do a translate search of our target string.
+    $search = [
+      'string' => $default_string,
+      'langcode' => $langcode_browser_fallback,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $language_browser_fallback_string,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    $search = [
+      'string' => $default_string,
+      'langcode' => $langcode,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $language_string,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Configure selected language negotiation to use zh-hans.
+    $edit = ['selected_langcode' => $langcode];
+    $this->drupalGet('admin/config/regional/language/detection/selected');
+    $this->submitForm($edit, 'Save configuration');
+    $test = [
+      'language_negotiation' => [LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $language_string,
+      'expected_method_id' => LanguageNegotiationSelected::METHOD_ID,
+      'http_header' => $http_header_browser_fallback,
+      'message' => 'SELECTED: UI language is switched based on selected language.',
+    ];
+    $this->doRunTest($test);
+
+    // An invalid language is selected.
+    $this->config('language.negotiation')->set('selected_langcode', NULL)->save();
+    $test = [
+      'language_negotiation' => [LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => $http_header_browser_fallback,
+      'message' => 'SELECTED > DEFAULT: UI language is switched based on selected language.',
+    ];
+    $this->doRunTest($test);
+
+    // No selected language is available.
+    $this->config('language.negotiation')->set('selected_langcode', $langcode_unknown)->save();
+    $test = [
+      'language_negotiation' => [LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => $http_header_browser_fallback,
+      'message' => 'SELECTED > DEFAULT: UI language is switched based on selected language.',
+    ];
+    $this->doRunTest($test);
+
+    $tests = [
+      // Default, browser preference should have no influence.
+      [
+        'language_negotiation' => [LanguageNegotiationUrl::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+        'path' => 'admin/config',
+        'expect' => $default_string,
+        'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'URL (PATH) > DEFAULT: no language prefix, UI language is default and the browser language preference setting is not used.',
+      ],
+      // Language prefix.
+      [
+        'language_negotiation' => [LanguageNegotiationUrl::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+        'path' => "$langcode/admin/config",
+        'expect' => $language_string,
+        'expected_method_id' => LanguageNegotiationUrl::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'URL (PATH) > DEFAULT: with language prefix, UI language is switched based on path prefix',
+      ],
+      // Default, go by browser preference.
+      [
+        'language_negotiation' => [LanguageNegotiationUrl::METHOD_ID, LanguageNegotiationBrowser::METHOD_ID],
+        'path' => 'admin/config',
+        'expect' => $language_browser_fallback_string,
+        'expected_method_id' => LanguageNegotiationBrowser::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'URL (PATH) > BROWSER: no language prefix, UI language is determined by browser language preference',
+      ],
+      // Prefix, switch to the language.
+      [
+        'language_negotiation' => [LanguageNegotiationUrl::METHOD_ID, LanguageNegotiationBrowser::METHOD_ID],
+        'path' => "$langcode/admin/config",
+        'expect' => $language_string,
+        'expected_method_id' => LanguageNegotiationUrl::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'URL (PATH) > BROWSER: with language prefix, UI language is based on path prefix',
+      ],
+      // Default, browser language preference is not one of site's lang.
+      [
+        'language_negotiation' => [LanguageNegotiationUrl::METHOD_ID, LanguageNegotiationBrowser::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+        'path' => 'admin/config',
+        'expect' => $default_string,
+        'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+        'http_header' => $http_header_blah,
+        'message' => 'URL (PATH) > BROWSER > DEFAULT: no language prefix and browser language preference set to unknown language should use default language',
+      ],
+    ];
+
+    foreach ($tests as $test) {
+      $this->doRunTest($test);
+    }
+
+    // Unknown language prefix should return 404.
+    $definitions = \Drupal::languageManager()->getNegotiator()->getNegotiationMethods();
+    // Enable only methods, which are either not limited to a specific language
+    // type or are supporting the interface language type.
+    $language_interface_method_definitions = array_filter($definitions, function ($method_definition) {
+      return !isset($method_definition['types']) || (isset($method_definition['types']) && in_array(LanguageInterface::TYPE_INTERFACE, $method_definition['types']));
+    });
+    $this->config('language.types')
+      ->set('negotiation.' . LanguageInterface::TYPE_INTERFACE . '.enabled', array_flip(array_keys($language_interface_method_definitions)))
+      ->save();
+    $this->drupalGet("$langcode_unknown/admin/config", [], $http_header_browser_fallback);
+    $this->assertSession()->statusCodeEquals(404);
+
+    // Set preferred langcode for user to NULL.
+    $account = $this->loggedInUser;
+    $account->preferred_langcode = NULL;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUser::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER > DEFAULT: no preferred user language setting, the UI language is default',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred langcode for user to default langcode.
+    $account = $this->loggedInUser;
+    $account->preferred_langcode = $default_language->getId();
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUser::METHOD_ID, LanguageNegotiationUrl::METHOD_ID],
+      'path' => "$langcode/admin/config",
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiationUser::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER > URL: User has default language as preferred user language setting, the UI language is default',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred langcode for user to unknown language.
+    $account = $this->loggedInUser;
+    $account->preferred_langcode = $langcode_unknown;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUser::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER > DEFAULT: invalid preferred user language setting, the UI language is default',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred langcode for user to non default.
+    $account->preferred_langcode = $langcode;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUser::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $language_string,
+      'expected_method_id' => LanguageNegotiationUser::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER > DEFAULT: defined preferred user language setting, the UI language is based on user setting',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred admin langcode for user to NULL.
+    $account->preferred_admin_langcode = NULL;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUserAdmin::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER ADMIN > DEFAULT: no preferred user admin language setting, the UI language is default',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred admin langcode for user to unknown language.
+    $account->preferred_admin_langcode = $langcode_unknown;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUserAdmin::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $default_string,
+      'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER ADMIN > DEFAULT: invalid preferred user admin language setting, the UI language is default',
+    ];
+    $this->doRunTest($test);
+
+    // Set preferred admin langcode for user to non default.
+    $account->preferred_admin_langcode = $langcode;
+    $account->save();
+
+    $test = [
+      'language_negotiation' => [LanguageNegotiationUserAdmin::METHOD_ID, LanguageNegotiationSelected::METHOD_ID],
+      'path' => 'admin/config',
+      'expect' => $language_string,
+      'expected_method_id' => LanguageNegotiationUserAdmin::METHOD_ID,
+      'http_header' => [],
+      'message' => 'USER ADMIN > DEFAULT: defined preferred user admin language setting, the UI language is based on user setting',
+    ];
+    $this->doRunTest($test);
+
+    // Go by session preference.
+    $language_negotiation_session_param = $this->randomMachineName();
+    $edit = ['language_negotiation_session_param' => $language_negotiation_session_param];
+    $this->drupalGet('admin/config/regional/language/detection/session');
+    $this->submitForm($edit, 'Save configuration');
+    $tests = [
+      [
+        'language_negotiation' => [LanguageNegotiationSession::METHOD_ID],
+        'path' => "admin/config",
+        'expect' => $default_string,
+        'expected_method_id' => LanguageNegotiatorInterface::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'SESSION > DEFAULT: no language given, the UI language is default',
+      ],
+      [
+        'language_negotiation' => [LanguageNegotiationSession::METHOD_ID],
+        'path' => 'admin/config',
+        'path_options' => ['query' => [$language_negotiation_session_param => $langcode]],
+        'expect' => $language_string,
+        'expected_method_id' => LanguageNegotiationSession::METHOD_ID,
+        'http_header' => $http_header_browser_fallback,
+        'message' => 'SESSION > DEFAULT: language given, UI language is determined by session language preference',
+      ],
+    ];
+    foreach ($tests as $test) {
+      $this->doRunTest($test);
+    }
+  }
+
+  protected function doRunTest($test) {
+    $test += ['path_options' => []];
+    if (!empty($test['language_negotiation'])) {
+      $method_weights = array_flip($test['language_negotiation']);
+      $this->container->get('language_negotiator')->saveConfiguration(LanguageInterface::TYPE_INTERFACE, $method_weights);
+    }
+    if (!empty($test['language_negotiation_url_part'])) {
+      $this->config('language.negotiation')
+        ->set('url.source', $test['language_negotiation_url_part'])
+        ->save();
+    }
+    if (!empty($test['language_test_domain'])) {
+      \Drupal::state()->set('language_test.domain', $test['language_test_domain']);
+    }
+    $this->container->get('language_manager')->reset();
+    $this->drupalGet($test['path'], $test['path_options'], $test['http_header']);
+    $this->assertSession()->pageTextContains($test['expect']);
+    $this->assertSession()->statusMessageContains('Language negotiation method: ' . $test['expected_method_id'], 'status');
+
+    // Get the private file and ensure it is a 200. It is important to
+    // invalidate the router cache to ensure the routing system runs a full
+    // match.
+    Cache::invalidateTags(['route_match']);
+    $this->drupalGet('system/files/test/private-file-test.txt');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
+  /**
+   * Tests URL language detection when the requested URL has no language.
+   */
+  public function testUrlLanguageFallback(): void {
+    // Add the Italian language.
+    $langcode_browser_fallback = 'it';
+    ConfigurableLanguage::createFromLangcode($langcode_browser_fallback)->save();
+    $languages = $this->container->get('language_manager')->getLanguages();
+
+    // Enable the path prefix for the default language: this way any unprefixed
+    // URL must have a valid fallback value.
+    $edit = ['prefix[en]' => 'en'];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Enable browser and URL language detection.
+    $edit = [
+      'language_interface[enabled][language-browser]' => TRUE,
+      'language_interface[enabled][language-url]' => TRUE,
+      'language_interface[weight][language-browser]' => -8,
+      'language_interface[weight][language-url]' => -10,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+    $this->drupalGet('admin/config/regional/language/detection');
+
+    // Enable the language switcher block.
+    $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_INTERFACE, ['id' => 'test_language_block']);
+
+    // Log out, because for anonymous users, the "active" class is set by PHP
+    // (which means we can easily test it here), whereas for authenticated users
+    // it is set by JavaScript.
+    $this->drupalLogout();
+
+    // Place a site branding block in the header region.
+    $this->drupalPlaceBlock('system_branding_block', [
+      'region' => 'header',
+      'id' => 'site_branding',
+    ]);
+
+    // Access the front page without specifying any valid URL language prefix
+    // and having as browser language preference a non-default language.
+    $http_header = ["Accept-Language" => "$langcode_browser_fallback;q=1"];
+    $language = new Language(['id' => '']);
+    $this->drupalGet('', ['language' => $language], $http_header);
+
+    // Check that the language switcher active link matches the given browser
+    // language.
+    $href = Url::fromRoute('<front>')->toString() . $langcode_browser_fallback;
+    $this->assertSession()->elementTextEquals('xpath', "//div[@id='block-test-language-block']//a[@class='language-link is-active' and starts-with(@href, '$href')]", $languages[$langcode_browser_fallback]->getName());
+
+    // Check that URLs are rewritten using the given browser language.
+    $this->assertSession()->elementTextEquals('xpath', "//div[@id='block-site-branding']/a[@rel='home' and @href='$href'][2]", 'Drupal');
+  }
+
+  /**
+   * Tests URL handling when separate domains are used for multiple languages.
+   */
+  public function testLanguageDomain(): void {
+    global $base_url;
+
+    // Get the current host URI we're running on.
+    $base_url_host = parse_url($base_url, PHP_URL_HOST);
+
+    // Add the Italian language.
+    ConfigurableLanguage::createFromLangcode('it')->save();
+
+    $languages = $this->container->get('language_manager')->getLanguages();
+
+    // Enable browser and URL language detection.
+    $edit = [
+      'language_interface[enabled][language-url]' => TRUE,
+      'language_interface[weight][language-url]' => -10,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Do not allow blank domain.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => '',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The domain may not be left blank for English', 'error');
+    $this->rebuildContainer();
+
+    // Change the domain for the Italian language.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => $base_url_host,
+      'domain[it]' => 'it.example.com',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains('The configuration options have been saved', 'status');
+    $this->rebuildContainer();
+
+    // Try to use an invalid domain.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => $base_url_host,
+      'domain[it]' => 'it.example.com/',
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    $this->assertSession()->statusMessageContains("The domain for Italian may only contain the domain name, not a trailing slash, protocol and/or port.", 'error');
+
+    // Build the link we're going to test.
+    $link = 'it.example.com' . rtrim(base_path(), '/') . '/admin';
+
+    // Test URL in another language: http://it.example.com/admin.
+    // Base path gives problems on the testbot, so $correct_link is hard-coded.
+    // @see UrlAlterFunctionalTest::assertUrlOutboundAlter (path.test).
+    $italian_url = Url::fromRoute('system.admin', [], ['language' => $languages['it']])->toString();
+    $url_scheme = \Drupal::request()->isSecure() ? 'https://' : 'http://';
+    $correct_link = $url_scheme . $link;
+    $this->assertEquals($correct_link, $italian_url, "The right URL ($italian_url) in accordance with the chosen language");
+
+    // Test HTTPS via options.
+    $italian_url = Url::fromRoute('system.admin', [], ['https' => TRUE, 'language' => $languages['it']])->toString();
+    $correct_link = 'https://' . $link;
+    $this->assertSame($correct_link, $italian_url, "The right HTTPS URL (via options) ($italian_url) in accordance with the chosen language");
+
+    // Test HTTPS via current URL scheme.
+    $request = Request::create('', 'GET', [], [], [], ['HTTPS' => 'on']);
+    $request->setSession(new Session(new MockArraySessionStorage()));
+    $this->container->get('request_stack')->push($request);
+    $italian_url = Url::fromRoute('system.admin', [], ['language' => $languages['it']])->toString();
+    $correct_link = 'https://' . $link;
+    $this->assertSame($correct_link, $italian_url, "The right URL (via current URL scheme) ($italian_url) in accordance with the chosen language");
+  }
+
+  /**
+   * Tests persistence of negotiation settings for the content language type.
+   */
+  public function testContentCustomization(): void {
+    // Customize content language settings from their defaults.
+    $edit = [
+      'language_content[configurable]' => TRUE,
+      'language_content[enabled][language-url]' => FALSE,
+      'language_content[enabled][language-session]' => TRUE,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Check if configurability persisted.
+    $config = $this->config('language.types');
+    $this->assertContains('language_interface', $config->get('configurable'), 'Interface language is configurable.');
+    $this->assertContains('language_content', $config->get('configurable'), 'Content language is configurable.');
+
+    // Ensure configuration was saved.
+    $this->assertArrayNotHasKey('language-url', $config->get('negotiation.language_content.enabled'));
+    $this->assertArrayHasKey('language-session', $config->get('negotiation.language_content.enabled'));
+  }
+
+  /**
+   * Tests if the language switcher block gets deleted when a language type has been made not configurable.
+   */
+  public function testDisableLanguageSwitcher(): void {
+    $block_id = 'test_language_block';
+
+    // Enable the language switcher block.
+    $this->drupalPlaceBlock('language_block:' . LanguageInterface::TYPE_CONTENT, ['id' => $block_id]);
+
+    // Check if the language switcher block has been created.
+    $block = Block::load($block_id);
+    $this->assertNotEmpty($block, 'Language switcher block was created.');
+
+    // Make sure language_content is not configurable.
+    $edit = [
+      'language_content[configurable]' => FALSE,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+    $this->assertSession()->statusCodeEquals(200);
+
+    // Check if the language switcher block has been removed.
+    $block = Block::load($block_id);
+    $this->assertNull($block, 'Language switcher block was removed.');
+  }
+
+}

core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php

@@ -0,0 +1,172 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional;
+
+use Drupal\Core\Language\Language;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\BrowserTestBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Tests that URL rewriting works as expected.
+ *
+ * @group language
+ */
+class LanguageUrlRewritingTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'language_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * A user with permissions to administer languages.
+   *
+   * @var \Drupal\user\UserInterface
+   */
+  protected $webUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Create and log in user.
+    $this->webUser = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($this->webUser);
+
+    // Install French language.
+    $edit = [];
+    $edit['predefined_langcode'] = 'fr';
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Enable URL language detection and selection.
+    $edit = ['language_interface[enabled][language-url]' => 1];
+    $this->drupalGet('admin/config/regional/language/detection');
+    $this->submitForm($edit, 'Save settings');
+
+    // Check that drupalSettings contains path prefix.
+    $this->drupalGet('fr/admin/config/regional/language/detection');
+    $this->assertSession()->responseContains('"pathPrefix":"fr\/"');
+  }
+
+  /**
+   * Check that non-installed languages are not considered.
+   */
+  public function testUrlRewritingEdgeCases(): void {
+    // Check URL rewriting with a non-installed language.
+    $non_existing = new Language(['id' => $this->randomMachineName()]);
+    $this->checkUrl($non_existing, 'Path language is ignored if language is not installed.');
+
+    // Check that URL rewriting is not applied to subrequests.
+    $this->drupalGet('language_test/subrequest');
+    $this->assertSession()->pageTextContains($this->webUser->getAccountName());
+  }
+
+  /**
+   * Check URL rewriting for the given language.
+   *
+   * The test is performed with a fixed URL (the default front page) to simply
+   * check that language prefixes are not added to it and that the prefixed URL
+   * is actually not working.
+   *
+   * @param \Drupal\Core\Language\LanguageInterface $language
+   *   The language object.
+   * @param string $message
+   *   Message to display in assertion that language prefixes are not added.
+   */
+  private function checkUrl(LanguageInterface $language, $message) {
+    $options = ['language' => $language, 'script' => ''];
+    $base_path = trim(base_path(), '/');
+    $rewritten_path = trim(str_replace($base_path, '', Url::fromRoute('<front>', [], $options)->toString()), '/');
+    $segments = explode('/', $rewritten_path, 2);
+    $prefix = $segments[0];
+    $path = $segments[1] ?? $prefix;
+
+    // If the rewritten URL has not a language prefix we pick a random prefix so
+    // we can always check the prefixed URL.
+    $prefixes = $this->config('language.negotiation')->get('url.prefixes');
+    $stored_prefix = $prefixes[$language->getId()] ?? $this->randomMachineName();
+    $this->assertNotEquals($prefix, $stored_prefix, $message);
+    $prefix = $stored_prefix;
+
+    $this->drupalGet("$prefix/$path");
+    $this->assertSession()->statusCodeEquals(404);
+  }
+
+  /**
+   * Check URL rewriting when using a domain name and a non-standard port.
+   */
+  public function testDomainNameNegotiationPort(): void {
+    global $base_url;
+    $language_domain = 'example.fr';
+    // Get the current host URI we're running on.
+    $base_url_host = parse_url($base_url, PHP_URL_HOST);
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => $base_url_host,
+      'domain[fr]' => $language_domain,
+    ];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+    // Rebuild the container so that the new language gets picked up by services
+    // that hold the list of languages.
+    $this->rebuildContainer();
+
+    // Enable domain configuration.
+    $this->config('language.negotiation')
+      ->set('url.source', LanguageNegotiationUrl::CONFIG_DOMAIN)
+      ->save();
+
+    // Reset static caching.
+    $this->container->get('language_manager')->reset();
+
+    // In case index.php is part of the URLs, we need to adapt the asserted
+    // URLs as well.
+    $index_php = str_contains(Url::fromRoute('<front>', [], ['absolute' => TRUE])->toString(), 'index.php');
+
+    $request = Request::createFromGlobals();
+    $server = $request->server->all();
+    $request = $this->prepareRequestForGenerator(TRUE, ['HTTP_HOST' => $server['HTTP_HOST'] . ':88']);
+
+    // Create an absolute French link.
+    $language = \Drupal::languageManager()->getLanguage('fr');
+    $url = Url::fromRoute('<front>', [], [
+      'absolute' => TRUE,
+      'language' => $language,
+    ])->toString();
+
+    $expected = ($index_php ? 'http://example.fr:88/index.php' : 'http://example.fr:88') . rtrim(base_path(), '/') . '/';
+
+    $this->assertEquals($expected, $url, 'The right port is used.');
+
+    // If we set the port explicitly, it should not be overridden.
+    $url = Url::fromRoute('<front>', [], [
+      'absolute' => TRUE,
+      'language' => $language,
+      'base_url' => $request->getBaseUrl() . ':90',
+    ])->toString();
+
+    $expected = $index_php ? 'http://example.fr:90/index.php' : 'http://example.fr:90' . rtrim(base_path(), '/') . '/';
+
+    $this->assertEquals($expected, $url, 'A given port is not overridden.');
+
+  }
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageJsonAnonTest.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageJsonAnonTest extends ConfigurableLanguageResourceTestBase {
+
+  use AnonResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageJsonBasicAuthTest.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageJsonBasicAuthTest extends ConfigurableLanguageResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageJsonCookieTest.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageJsonCookieTest extends ConfigurableLanguageResourceTestBase {
+
+  use CookieResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageResourceTestBase.php

@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Url;
+use Drupal\Tests\rest\Functional\EntityResource\ConfigEntityResourceTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+abstract class ConfigurableLanguageResourceTestBase extends ConfigEntityResourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $entityTypeId = 'configurable_language';
+
+  /**
+   * @var \Drupal\language\ConfigurableLanguageInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUpAuthorization($method) {
+    $this->grantPermissionsToTestedRole(['administer languages']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function createEntity() {
+    $configurable_language = ConfigurableLanguage::create([
+      'id' => 'll',
+      'label' => 'Llama Language',
+    ]);
+    $configurable_language->save();
+
+    return $configurable_language;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedNormalizedEntity() {
+    return [
+      'dependencies' => [],
+      'direction' => 'ltr',
+      'id' => 'll',
+      'label' => 'Llama Language',
+      'langcode' => 'en',
+      'locked' => FALSE,
+      'status' => TRUE,
+      'uuid' => $this->entity->uuid(),
+      'weight' => 0,
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedCacheContexts() {
+    return Cache::mergeContexts(parent::getExpectedCacheContexts(), ['languages:language_interface']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getNormalizedPostEntity() {
+    // @todo Update in https://www.drupal.org/node/2300677.
+    return [];
+  }
+
+  /**
+   * Tests a GET request for a default config entity, which has a _core key.
+   *
+   * @see https://www.drupal.org/node/2915414
+   */
+  public function testGetDefaultConfig(): void {
+    $this->initAuthentication();
+    $url = Url::fromUri('base:/entity/configurable_language/en')->setOption('query', ['_format' => static::$format]);
+    $request_options = $this->getAuthenticationRequestOptions('GET');
+    $this->provisionEntityResource();
+    $this->setUpAuthorization('GET');
+    $response = $this->request('GET', $url, $request_options);
+
+    $normalization = $this->serializer->decode((string) $response->getBody(), static::$format);
+    $this->assertArrayNotHasKey('_core', $normalization);
+  }
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageXmlAnonTest.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageXmlAnonTest extends ConfigurableLanguageResourceTestBase {
+
+  use AnonResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageXmlBasicAuthTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageXmlBasicAuthTest extends ConfigurableLanguageResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}

core/modules/language/tests/src/Functional/Rest/ConfigurableLanguageXmlCookieTest.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ConfigurableLanguageXmlCookieTest extends ConfigurableLanguageResourceTestBase {
+
+  use CookieResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsJsonAnonTest.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsJsonAnonTest extends ContentLanguageSettingsResourceTestBase {
+
+  use AnonResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsJsonBasicAuthTest.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsJsonBasicAuthTest extends ContentLanguageSettingsResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsJsonCookieTest.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsJsonCookieTest extends ContentLanguageSettingsResourceTestBase {
+
+  use CookieResourceTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'application/json';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsResourceTestBase.php

@@ -0,0 +1,95 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\node\Entity\NodeType;
+use Drupal\Tests\rest\Functional\EntityResource\ConfigEntityResourceTestBase;
+
+abstract class ContentLanguageSettingsResourceTestBase extends ConfigEntityResourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'node'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $entityTypeId = 'language_content_settings';
+
+  /**
+   * @var \Drupal\language\ContentLanguageSettingsInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUpAuthorization($method) {
+    $this->grantPermissionsToTestedRole(['administer languages']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function createEntity() {
+    // Create a "Camelids" node type.
+    $camelids = NodeType::create([
+      'name' => 'Camelids',
+      'type' => 'camelids',
+    ]);
+    $camelids->save();
+
+    $entity = ContentLanguageSettings::create([
+      'target_entity_type_id' => 'node',
+      'target_bundle' => 'camelids',
+    ]);
+    $entity->setDefaultLangcode('site_default')
+      ->save();
+
+    return $entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedNormalizedEntity() {
+    return [
+      'default_langcode' => 'site_default',
+      'dependencies' => [
+        'config' => [
+          'node.type.camelids',
+        ],
+      ],
+      'id' => 'node.camelids',
+      'langcode' => 'en',
+      'language_alterable' => FALSE,
+      'status' => TRUE,
+      'target_bundle' => 'camelids',
+      'target_entity_type_id' => 'node',
+      'uuid' => $this->entity->uuid(),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getNormalizedPostEntity() {
+    // @todo Update in https://www.drupal.org/node/2300677.
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedCacheContexts() {
+    return [
+      'languages:language_interface',
+      'user.permissions',
+    ];
+  }
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsXmlAnonTest.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsXmlAnonTest extends ContentLanguageSettingsResourceTestBase {
+
+  use AnonResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsXmlBasicAuthTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsXmlBasicAuthTest extends ContentLanguageSettingsResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}

core/modules/language/tests/src/Functional/Rest/ContentLanguageSettingsXmlCookieTest.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Functional\Rest;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class ContentLanguageSettingsXmlCookieTest extends ContentLanguageSettingsResourceTestBase {
+
+  use CookieResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+}

core/modules/language/tests/src/Kernel/Condition/LanguageConditionTest.php

@@ -0,0 +1,101 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Condition;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the language condition plugin.
+ *
+ * @group language
+ */
+class LanguageConditionTest extends KernelTestBase {
+
+  /**
+   * The condition plugin manager.
+   *
+   * @var \Drupal\Core\Condition\ConditionManager
+   */
+  protected $manager;
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['system', 'language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installConfig(['language']);
+    // Setup Italian.
+    ConfigurableLanguage::createFromLangcode('it')->save();
+
+    $this->manager = $this->container->get('plugin.manager.condition');
+  }
+
+  /**
+   * Tests the language condition.
+   */
+  public function testConditions(): void {
+    // Grab the language condition and configure it to check the content
+    // language.
+    $language = \Drupal::languageManager()->getLanguage('en');
+    $condition = $this->manager->createInstance('language')
+      ->setConfig('langcodes', ['en' => 'en', 'it' => 'it'])
+      ->setContextValue('language', $language);
+    $this->assertTrue($condition->execute(), 'Language condition passes as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is English, Italian.', $condition->summary());
+
+    // Change to Italian only.
+    $condition->setConfig('langcodes', ['it' => 'it']);
+    $this->assertFalse($condition->execute(), 'Language condition fails as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is Italian.', $condition->summary());
+
+    // Negate the condition
+    $condition->setConfig('negate', TRUE);
+    $this->assertTrue($condition->execute(), 'Language condition passes as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is not Italian.', $condition->summary());
+
+    // Change the default language to Italian.
+    $language = \Drupal::languageManager()->getLanguage('it');
+
+    $condition = $this->manager->createInstance('language')
+      ->setConfig('langcodes', ['en' => 'en', 'it' => 'it'])
+      ->setContextValue('language', $language);
+
+    $this->assertTrue($condition->execute(), 'Language condition passes as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is English, Italian.', $condition->summary());
+
+    // Change to Italian only.
+    $condition->setConfig('langcodes', ['it' => 'it']);
+    $this->assertTrue($condition->execute(), 'Language condition passes as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is Italian.', $condition->summary());
+
+    // Negate the condition
+    $condition->setConfig('negate', TRUE);
+    $this->assertFalse($condition->execute(), 'Language condition fails as expected.');
+    // Check for the proper summary.
+    $this->assertEquals('The language is not Italian.', $condition->summary());
+  }
+
+}

core/modules/language/tests/src/Kernel/ConfigurableLanguageManagerTest.php

@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+
+/**
+ * Tests the ConfigurableLanguage entity.
+ *
+ * @group language
+ * @coversDefaultClass \Drupal\language\ConfigurableLanguageManager
+ */
+class ConfigurableLanguageManagerTest extends LanguageTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['user'];
+
+  /**
+   * The language negotiator.
+   *
+   * @var \Drupal\language\LanguageNegotiatorInterface
+   */
+  protected $languageNegotiator;
+
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('user');
+
+    $this->languageNegotiator = $this->container->get('language_negotiator');
+    $this->languageManager = $this->container->get('language_manager');
+  }
+
+  /**
+   * @covers ::getLanguageSwitchLinks
+   */
+  public function testLanguageSwitchLinks(): void {
+    $this->languageNegotiator->setCurrentUser($this->prophesize('Drupal\Core\Session\AccountInterface')->reveal());
+    $this->languageManager->getLanguageSwitchLinks(LanguageInterface::TYPE_INTERFACE, new Url('<current>'));
+  }
+
+}

core/modules/language/tests/src/Kernel/ConfigurableLanguageTest.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests the ConfigurableLanguage entity.
+ *
+ * @group language
+ * @see \Drupal\language\Entity\ConfigurableLanguage.
+ */
+class ConfigurableLanguageTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Tests configurable language name methods.
+   */
+  public function testName(): void {
+    $name = $this->randomMachineName();
+    $language_code = $this->randomMachineName(2);
+    $configurableLanguage = new ConfigurableLanguage(['label' => $name, 'id' => $language_code], 'configurable_language');
+    $this->assertEquals($name, $configurableLanguage->getName());
+    $this->assertEquals('Test language', $configurableLanguage->setName('Test language')->getName());
+  }
+
+}

core/modules/language/tests/src/Kernel/ConfigurableLanguageValidationTest.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\KernelTests\Core\Config\ConfigEntityValidationTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests validation of configurable_language entities.
+ *
+ * @group language
+ * @group #slow
+ */
+class ConfigurableLanguageValidationTest extends ConfigEntityValidationTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->entity = ConfigurableLanguage::createFromLangcode('fr');
+    $this->entity->save();
+  }
+
+}

core/modules/language/tests/src/Kernel/ContentLanguageSettingsValidationTest.php

@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\entity_test\Entity\EntityTestBundle;
+use Drupal\KernelTests\Core\Config\ConfigEntityValidationTestBase;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\node\Traits\ContentTypeCreationTrait;
+
+/**
+ * Tests validation of content_language_settings entities.
+ *
+ * @group language
+ * @group #slow
+ */
+class ContentLanguageSettingsValidationTest extends ConfigEntityValidationTestBase {
+
+  use ContentTypeCreationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'entity_test',
+    'field',
+    'language',
+    'node',
+    'text',
+    'user',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected bool $hasLabel = FALSE;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installConfig('node');
+
+    $this->createContentType(['type' => 'alpha']);
+    $this->createContentType(['type' => 'bravo']);
+
+    EntityTestBundle::create(['id' => 'alpha'])->save();
+    EntityTestBundle::create(['id' => 'bravo'])->save();
+
+    $this->entity = ContentLanguageSettings::create([
+      'target_entity_type_id' => 'node',
+      'target_bundle' => 'alpha',
+    ]);
+    $this->entity->save();
+  }
+
+  /**
+   * Tests that the target bundle of the language content settings is checked.
+   */
+  public function testTargetBundleMustExist(): void {
+    $this->entity->set('target_bundle', 'superhero');
+    $this->assertValidationErrors([
+      '' => "The 'target_bundle' property cannot be changed.",
+      'target_bundle' => "The 'superhero' bundle does not exist on the 'node' entity type.",
+    ]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testImmutableProperties(array $valid_values = []): void {
+    parent::testImmutableProperties([
+      'target_entity_type_id' => 'entity_test_with_bundle',
+      'target_bundle' => 'bravo',
+    ]);
+  }
+
+}

core/modules/language/tests/src/Kernel/EntityDefaultLanguageTest.php

@@ -0,0 +1,151 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests default language code is properly generated for entities.
+ *
+ * @group language
+ */
+class EntityDefaultLanguageTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'language',
+    'node',
+    'field',
+    'text',
+    'user',
+    'system',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('user');
+
+    // Activate Spanish language, so there are two languages activated.
+    $language = $this->container->get('entity_type.manager')->getStorage('configurable_language')->create([
+      'id' => 'es',
+      'label' => 'Spanish',
+    ]);
+    $language->save();
+
+    // Create a new content type which has Undefined language by default.
+    $this->createContentType('content_und', LanguageInterface::LANGCODE_NOT_SPECIFIED);
+    // Create a new content type which has Spanish language by default.
+    $this->createContentType('content_es', 'es');
+  }
+
+  /**
+   * Tests that default language code is properly set for new nodes.
+   */
+  public function testEntityTranslationDefaultLanguageViaCode(): void {
+    // With language module activated, and a content type that is configured to
+    // have no language by default, a new node of this content type will have
+    // "und" language code when language is not specified.
+    $node = $this->createNode('content_und');
+    $this->assertEquals(LanguageInterface::LANGCODE_NOT_SPECIFIED, $node->langcode->value);
+    // With language module activated, and a content type that is configured to
+    // have no language by default, a new node of this content type will have
+    // "es" language code when language is specified as "es".
+    $node = $this->createNode('content_und', 'es');
+    $this->assertEquals('es', $node->langcode->value);
+
+    // With language module activated, and a content type that is configured to
+    // have language "es" by default, a new node of this content type will have
+    // "es" language code when language is not specified.
+    $node = $this->createNode('content_es');
+    $this->assertEquals('es', $node->langcode->value);
+    // With language module activated, and a content type that is configured to
+    // have language "es" by default, a new node of this content type will have
+    // "en" language code when language "en" is specified.
+    $node = $this->createNode('content_es', 'en');
+    $this->assertEquals('en', $node->langcode->value);
+
+    // Disable language module.
+    $this->disableModules(['language']);
+
+    // With language module disabled, and a content type that is configured to
+    // have no language specified by default, a new node of this content type
+    // will have site's default language code when language is not specified.
+    $node = $this->createNode('content_und');
+    $this->assertEquals('en', $node->langcode->value);
+    // With language module disabled, and a content type that is configured to
+    // have no language specified by default, a new node of this type will have
+    // "es" language code when language "es" is specified.
+    $node = $this->createNode('content_und', 'es');
+    $this->assertEquals('es', $node->langcode->value);
+
+    // With language module disabled, and a content type that is configured to
+    // have language "es" by default, a new node of this type will have site's
+    // default language code when language is not specified.
+    $node = $this->createNode('content_es');
+    $this->assertEquals('en', $node->langcode->value);
+    // With language module disabled, and a content type that is configured to
+    // have language "es" by default, a new node of this type will have "en"
+    // language code when language "en" is specified.
+    $node = $this->createNode('content_es', 'en');
+    $this->assertEquals('en', $node->langcode->value);
+  }
+
+  /**
+   * Creates a new node content type.
+   *
+   * @param string $name
+   *   The content type name.
+   * @param string $langcode
+   *   Default language code of the nodes of this type.
+   */
+  protected function createContentType($name, $langcode) {
+    $content_type = $this->container->get('entity_type.manager')->getStorage('node_type')->create([
+      'name' => 'Test ' . $name,
+      'title_label' => 'Title',
+      'type' => $name,
+      'create_body' => FALSE,
+    ]);
+    $content_type->save();
+    ContentLanguageSettings::loadByEntityTypeBundle('node', $name)
+      ->setLanguageAlterable(FALSE)
+      ->setDefaultLangcode($langcode)
+      ->save();
+
+  }
+
+  /**
+   * Creates a new node of given type and language using Entity API.
+   *
+   * @param string $type
+   *   The node content type.
+   * @param string $langcode
+   *   (optional) Language code to pass to entity create.
+   *
+   * @return \Drupal\node\NodeInterface
+   *   The node created.
+   */
+  protected function createNode($type, $langcode = NULL) {
+    $values = [
+      'type' => $type,
+      'title' => $this->randomString(),
+    ];
+    if (!empty($langcode)) {
+      $values['langcode'] = $langcode;
+    }
+    $node = $this->container->get('entity_type.manager')->getStorage('node')->create($values);
+    return $node;
+  }
+
+}

core/modules/language/tests/src/Kernel/EntityUrlLanguageTest.php

@@ -0,0 +1,143 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationContentEntity;
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Core\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Session\Session;
+use Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Tests the language of entity URLs.
+ * @group language
+ */
+class EntityUrlLanguageTest extends LanguageTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['entity_test', 'user'];
+
+  /**
+   * The entity being used for testing.
+   *
+   * @var \Drupal\Core\Entity\ContentEntityInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('entity_test');
+    $this->installEntitySchema('configurable_language');
+
+    // In order to reflect the changes for a multilingual site in the container
+    // we have to rebuild it.
+    ConfigurableLanguage::createFromLangcode('es')->save();
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+
+    $config = $this->config('language.negotiation');
+    $config->set('url.prefixes', ['en' => 'en', 'es' => 'es', 'fr' => 'fr'])
+      ->save();
+
+    \Drupal::service('kernel')->rebuildContainer();
+
+    $this->createTranslatableEntity();
+  }
+
+  /**
+   * Ensures that entity URLs in a language have the right language prefix.
+   */
+  public function testEntityUrlLanguage(): void {
+    $this->assertStringContainsString('/en/entity_test/' . $this->entity->id(), $this->entity->toUrl()->toString());
+    $this->assertStringContainsString('/es/entity_test/' . $this->entity->id(), $this->entity->getTranslation('es')->toUrl()->toString());
+    $this->assertStringContainsString('/fr/entity_test/' . $this->entity->id(), $this->entity->getTranslation('fr')->toUrl()->toString());
+  }
+
+  /**
+   * Ensures correct entity URLs with the method language-content-entity enabled.
+   *
+   * Test case with the method language-content-entity enabled and configured
+   * with higher and also with lower priority than the method language-url.
+   */
+  public function testEntityUrlLanguageWithLanguageContentEnabled(): void {
+    // Define the method language-content-entity with a higher priority than
+    // language-url.
+    $config = $this->config('language.types');
+    $config->set('configurable', [LanguageInterface::TYPE_INTERFACE, LanguageInterface::TYPE_CONTENT]);
+    $config->set('negotiation.language_content.enabled', [
+      LanguageNegotiationContentEntity::METHOD_ID => 0,
+      LanguageNegotiationUrl::METHOD_ID => 1,
+    ]);
+    $config->save();
+
+    // Without being on a content entity route the default entity URL tests
+    // should still pass.
+    $this->testEntityUrlLanguage();
+
+    // Now switching to an entity route, so that the URL links are generated
+    // while being on an entity route.
+    $this->setCurrentRequestForRoute('/entity_test/{entity_test}', 'entity.entity_test.canonical');
+
+    // The method language-content-entity should run before language-url and
+    // append query parameter for the content language and prevent language-url
+    // from overwriting the URL.
+    $this->assertStringContainsString('/en/entity_test/' . $this->entity->id() . '?' . LanguageNegotiationContentEntity::QUERY_PARAMETER . '=en', $this->entity->toUrl('canonical')->toString());
+    $this->assertStringContainsString('/en/entity_test/' . $this->entity->id() . '?' . LanguageNegotiationContentEntity::QUERY_PARAMETER . '=es', $this->entity->getTranslation('es')->toUrl('canonical')->toString());
+    $this->assertStringContainsString('/en/entity_test/' . $this->entity->id() . '?' . LanguageNegotiationContentEntity::QUERY_PARAMETER . '=fr', $this->entity->getTranslation('fr')->toUrl('canonical')->toString());
+
+    // Define the method language-url with a higher priority than
+    // language-content-entity. This configuration should match the default one,
+    // where the language-content-entity is turned off.
+    $config->set('negotiation.language_content.enabled', [
+      LanguageNegotiationUrl::METHOD_ID => 0,
+      LanguageNegotiationContentEntity::METHOD_ID => 1,
+    ]);
+    $config->save();
+
+    // The default entity URL tests should pass again with the current
+    // configuration.
+    $this->testEntityUrlLanguage();
+  }
+
+  /**
+   * Creates a translated entity.
+   */
+  protected function createTranslatableEntity() {
+    $this->entity = EntityTest::create();
+    $this->entity->addTranslation('es', ['name' => 'name spanish']);
+    $this->entity->addTranslation('fr', ['name' => 'name french']);
+    $this->entity->save();
+  }
+
+  /**
+   * Sets the current request to a specific path with the corresponding route.
+   *
+   * @param string $path
+   *   The path for which the current request should be created.
+   * @param string $route_name
+   *   The route name for which the route object for the request should be
+   *   created.
+   */
+  protected function setCurrentRequestForRoute($path, $route_name) {
+    $request = Request::create($path);
+    $request->attributes->set(RouteObjectInterface::ROUTE_NAME, $route_name);
+    $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route($path));
+    $request->setSession(new Session(new MockArraySessionStorage()));
+    $this->container->get('request_stack')->push($request);
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageConfigFactoryOverrideTest.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests \Drupal\language\Config\LanguageConfigFactoryOverride.
+ *
+ * @group language
+ */
+class LanguageConfigFactoryOverrideTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['system', 'language'];
+
+  /**
+   * Tests language.config_factory_override service has the default language.
+   */
+  public function testLanguageConfigFactoryOverride(): void {
+    $this->installConfig('system');
+    $this->installConfig('language');
+
+    /** @var \Drupal\language\Config\LanguageConfigFactoryOverride $config_factory_override */
+    $config_factory_override = \Drupal::service('language.config_factory_override');
+    $this->assertEquals('en', $config_factory_override->getLanguage()->getId());
+
+    ConfigurableLanguage::createFromLangcode('de')->save();
+
+    // Invalidate the container.
+    $this->config('system.site')->set('default_langcode', 'de')->save();
+    $this->container->get('kernel')->rebuildContainer();
+
+    $config_factory_override = \Drupal::service('language.config_factory_override');
+    $this->assertEquals('de', $config_factory_override->getLanguage()->getId());
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageConfigOverrideInstallTest.php

@@ -0,0 +1,93 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Config\ConfigCollectionEvents;
+use Drupal\language\Config\LanguageConfigOverrideEvents;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\KernelTests\KernelTestBase;
+
+// cspell:ignore deutsch
+
+/**
+ * Ensures the language config overrides can be installed.
+ *
+ * @group language
+ */
+class LanguageConfigOverrideInstallTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'config_events_test', 'language_events_test'];
+
+  /**
+   * Tests the configuration events are not fired during install of overrides.
+   */
+  public function testLanguageConfigOverrideInstall(): void {
+    $this->installConfig(['language']);
+    ConfigurableLanguage::createFromLangcode('de')->save();
+    // Need to enable test module after creating the language otherwise saving
+    // the language will install the configuration.
+    $this->enableModules(['language_config_override_test']);
+    \Drupal::state()->set('config_events_test.event', FALSE);
+    \Drupal::state()->set('language_events_test.all_events', []);
+    $this->installConfig(['language_config_override_test']);
+
+    // Ensure the save-in-collection event is triggered when saving data in
+    // config collections during an install.
+    $event_recorder = \Drupal::state()->get('config_events_test.event', FALSE);
+    $this->assertSame([
+      'event_name' => ConfigCollectionEvents::SAVE_IN_COLLECTION,
+      'current_config_data' => ['name' => 'Deutsch'],
+      'original_config_data' => [],
+      'raw_config_data' => ['name' => 'Deutsch'],
+    ], $event_recorder);
+    $config = \Drupal::service('language.config_factory_override')->getOverride('de', 'language_config_override_test.settings');
+    $this->assertEquals('Deutsch', $config->get('name'));
+
+    // Ensure the save override event is triggered when saving overrides during
+    // an install.
+    $event_recorder = \Drupal::state()->get('language_events_test.all_events', []);
+    $this->assertArrayHasKey(LanguageConfigOverrideEvents::SAVE_OVERRIDE, $event_recorder);
+    $this->assertArrayHasKey('language_config_override_test.settings', $event_recorder[LanguageConfigOverrideEvents::SAVE_OVERRIDE]);
+    $this->assertSame([
+      'event_name' => LanguageConfigOverrideEvents::SAVE_OVERRIDE,
+      'current_override_data' => ['name' => 'Deutsch'],
+      'original_override_data' => [],
+    ], $event_recorder['language.save_override']['language_config_override_test.settings'][0]);
+
+    // Test events during uninstall.
+    \Drupal::state()->set('config_events_test.all_events', []);
+    \Drupal::state()->set('language_events_test.all_events', []);
+    $this->container->get('module_installer')->uninstall(['language_config_override_test']);
+
+    // Ensure the delete-in-collection event is triggered when deleting data in
+    // config collections during an uninstall.
+    $event_recorder = \Drupal::state()->get('config_events_test.all_events', []);
+    $this->assertArrayHasKey(ConfigCollectionEvents::DELETE_IN_COLLECTION, $event_recorder);
+    $this->assertArrayHasKey('language_config_override_test.settings', $event_recorder[ConfigCollectionEvents::DELETE_IN_COLLECTION]);
+    $this->assertSame([
+      'event_name' => ConfigCollectionEvents::DELETE_IN_COLLECTION,
+      'current_config_data' => [],
+      'original_config_data' => ['name' => 'Deutsch'],
+      'raw_config_data' => [],
+    ], $event_recorder[ConfigCollectionEvents::DELETE_IN_COLLECTION]['language_config_override_test.settings'][0]);
+
+    // Ensure the delete override event is triggered when deleting overrides
+    // during an uninstall.
+    $event_recorder = \Drupal::state()->get('language_events_test.all_events', []);
+    $this->assertArrayHasKey(LanguageConfigOverrideEvents::DELETE_OVERRIDE, $event_recorder);
+    $this->assertArrayHasKey('language_config_override_test.settings', $event_recorder[LanguageConfigOverrideEvents::DELETE_OVERRIDE]);
+    $this->assertSame([
+      'event_name' => LanguageConfigOverrideEvents::DELETE_OVERRIDE,
+      'current_override_data' => [],
+      'original_override_data' => ['name' => 'Deutsch'],
+    ], $event_recorder['language.delete_override']['language_config_override_test.settings'][0]);
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageDependencyInjectionTest.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Exception\DeleteDefaultLanguageException;
+
+/**
+ * Tests that a language object can be injected.
+ *
+ * @group language
+ */
+class LanguageDependencyInjectionTest extends LanguageTestBase {
+
+  /**
+   * Tests dependency injected languages against a new Language object.
+   *
+   * @see \Drupal\Core\Language\LanguageInterface
+   */
+  public function testDependencyInjectedNewLanguage(): void {
+    $expected = $this->languageManager->getDefaultLanguage();
+    $result = $this->languageManager->getCurrentLanguage();
+    $this->assertSame($expected, $result);
+  }
+
+  /**
+   * Tests dependency injected Language object.
+   *
+   * @see \Drupal\Core\Language\Language
+   */
+  public function testDependencyInjectedNewDefaultLanguage(): void {
+    $default_language = ConfigurableLanguage::load(\Drupal::languageManager()->getDefaultLanguage()->getId());
+    // Change the language default object to different values.
+    $fr = ConfigurableLanguage::createFromLangcode('fr');
+    $fr->save();
+    $this->config('system.site')->set('default_langcode', 'fr')->save();
+
+    // The language system creates a Language object which contains the
+    // same properties as the new default language object.
+    $result = \Drupal::languageManager()->getCurrentLanguage();
+    $this->assertSame('fr', $result->getId());
+
+    // Delete the language to check that we fallback to the default.
+    try {
+      $fr->delete();
+      $this->fail('Expected DeleteDefaultLanguageException thrown.');
+    }
+    catch (DeleteDefaultLanguageException $e) {
+      // Expected exception; just continue testing.
+    }
+
+    // Re-save the previous default language and the delete should work.
+    $this->config('system.site')->set('default_langcode', $default_language->getId())->save();
+
+    $fr->delete();
+    $result = \Drupal::languageManager()->getCurrentLanguage();
+    $this->assertSame($default_language->getId(), $result->getId());
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageFallbackTest.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests the language fallback behavior.
+ *
+ * @group language
+ */
+class LanguageFallbackTest extends LanguageTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $i = 0;
+    foreach (['af', 'am', 'ar'] as $langcode) {
+      $language = ConfigurableLanguage::createFromLangcode($langcode);
+      $language->set('weight', $i--);
+      $language->save();
+    }
+  }
+
+  /**
+   * Tests language fallback candidates.
+   */
+  public function testCandidates(): void {
+    $language_list = $this->languageManager->getLanguages();
+    $expected = array_keys($language_list + [LanguageInterface::LANGCODE_NOT_SPECIFIED => NULL]);
+
+    // Check that language fallback candidates by default are all the available
+    // languages sorted by weight.
+    $candidates = $this->languageManager->getFallbackCandidates();
+    $this->assertEquals($expected, array_values($candidates), 'Language fallback candidates are properly returned.');
+
+    // Check that candidates are alterable.
+    $this->state->set('language_test.fallback_alter.candidates', TRUE);
+    $expected = array_slice($expected, 0, count($expected) - 1);
+    $candidates = $this->languageManager->getFallbackCandidates();
+    $this->assertEquals($expected, array_values($candidates), 'Language fallback candidates are alterable.');
+
+    // Check that candidates are alterable for specific operations.
+    $this->state->set('language_test.fallback_alter.candidates', FALSE);
+    $this->state->set('language_test.fallback_operation_alter.candidates', TRUE);
+    $expected[] = LanguageInterface::LANGCODE_NOT_SPECIFIED;
+    $expected[] = LanguageInterface::LANGCODE_NOT_APPLICABLE;
+    $candidates = $this->languageManager->getFallbackCandidates(['operation' => 'test']);
+    $this->assertEquals($expected, array_values($candidates), 'Language fallback candidates are alterable for specific operations.');
+
+    // Check that when the site is monolingual no language fallback is applied.
+    /** @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $configurable_language_storage */
+    $configurable_language_storage = $this->container->get('entity_type.manager')->getStorage('configurable_language');
+    foreach ($language_list as $langcode => $language) {
+      if (!$language->isDefault()) {
+        $configurable_language_storage->load($langcode)->delete();
+      }
+    }
+    $candidates = $this->languageManager->getFallbackCandidates();
+    $this->assertEquals([LanguageInterface::LANGCODE_DEFAULT], array_values($candidates), 'Language fallback is not applied when the Language module is not enabled.');
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageNegotiatorPluginTest.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Logger\LoggerChannelFactory;
+use Drupal\KernelTests\KernelTestBase;
+use Symfony\Component\ErrorHandler\BufferingLogger;
+
+/**
+ * Tests PluginNotFoundException.
+ *
+ * @group language
+ */
+class LanguageNegotiatorPluginTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'user'];
+
+  /**
+   * Tests for PluginNotFoundException.
+   */
+  public function testLanguageNegotiatorNoPlugin(): void {
+    $logger = new BufferingLogger();
+    $logger_factory = $this->createMock(LoggerChannelFactory::class);
+    $logger_factory->expects($this->once())
+      ->method('get')
+      ->with('language')
+      ->willReturn($logger);
+    $this->container->set('logger.factory', $logger_factory);
+    $this->installEntitySchema('user');
+
+    // Test unavailable plugin.
+    $config = $this->config('language.types');
+    $config->set('configurable', [LanguageInterface::TYPE_URL]);
+    $config->set('negotiation.language_url.enabled', [
+      self::CLASS => -3,
+    ]);
+    $config->save();
+    $languageNegotiator = $this->container->get('language_negotiator');
+    $languageNegotiator->setCurrentUser($this->prophesize('Drupal\Core\Session\AccountInterface')->reveal());
+    try {
+      $languageNegotiator->initializeType(LanguageInterface::TYPE_URL);
+    }
+    catch (PluginNotFoundException $exception) {
+      $this->fail('Plugin not found exception unhandled.');
+    }
+    $log_message = $logger->cleanLogs()[0];
+    $this->assertEquals('error', $log_message[0]);
+    $this->assertStringContainsString('The "Drupal\Tests\language\Kernel\LanguageNegotiatorPluginTest" plugin does not exist.', $log_message[1]);
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageSelectWidgetTest.php

@@ -0,0 +1,79 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the language select widget.
+ *
+ * @group language
+ */
+class LanguageSelectWidgetTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'entity_test',
+    'language',
+    'user',
+    'system',
+  ];
+
+  /**
+   * The entity form display.
+   *
+   * @var \Drupal\Core\Entity\Entity\EntityFormDisplay
+   */
+  protected $entityFormDisplay;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installEntitySchema('entity_test');
+    $this->installEntitySchema('user');
+
+    $storage = $this->container->get('entity_type.manager')->getStorage('entity_form_display');
+    $this->entityFormDisplay = $storage->create([
+      'targetEntityType' => 'entity_test',
+      'bundle' => 'entity_test',
+      'mode' => 'default',
+      'status' => TRUE,
+    ]);
+  }
+
+  /**
+   * Tests the widget with the locked languages.
+   */
+  public function testWithIncludedLockedLanguage(): void {
+    $this->entityFormDisplay->setComponent('langcode', [
+      'type' => 'language_select',
+    ])->save();
+    $entity = EntityTest::create(['name' => $this->randomString()]);
+    $form = $this->container->get('entity.form_builder')->getForm($entity);
+    $options = array_keys($form['langcode']['widget'][0]['value']['#options']);
+    $this->assertSame(['en', 'und', 'zxx'], $options);
+  }
+
+  /**
+   * Tests the widget without the locked languages.
+   */
+  public function testWithoutIncludedLockedLanguage(): void {
+    $this->entityFormDisplay->setComponent('langcode', [
+      'type' => 'language_select',
+      'settings' => ['include_locked' => FALSE],
+    ])->save();
+    $entity = EntityTest::create(['name' => $this->randomString()]);
+    $form = $this->container->get('entity.form_builder')->getForm($entity);
+    $options = array_keys($form['langcode']['widget'][0]['value']['#options']);
+    $this->assertSame(['en'], $options);
+  }
+
+}

core/modules/language/tests/src/Kernel/LanguageTestBase.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Test for dependency injected language object.
+ */
+abstract class LanguageTestBase extends KernelTestBase {
+
+  protected static $modules = ['system', 'language', 'language_test'];
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The state storage service.
+   *
+   * @var \Drupal\Core\State\StateInterface
+   */
+  protected $state;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installConfig(['language']);
+
+    $this->state = $this->container->get('state');
+
+    // Ensure we are building a new Language object for each test.
+    $this->languageManager = $this->container->get('language_manager');
+    $this->languageManager->reset();
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateDefaultLanguageTest.php

@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests the default language variable migration.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateDefaultLanguageTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Tests language_default migration with an existing language.
+   */
+  public function testMigrationWithExistingLanguage(): void {
+    $this->setDefaultLanguage('fr');
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    // Tests the language is loaded and is the default language.
+    $default_language = ConfigurableLanguage::load('fr');
+    $this->assertNotNull($default_language);
+    $this->assertSame('fr', $this->config('system.site')->get('default_langcode'));
+  }
+
+  /**
+   * Tests language_default migration with a non-existing language.
+   */
+  public function testMigrationWithNonExistentLanguage(): void {
+    $this->setDefaultLanguage('tv');
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    // Tests the migration log contains an error message.
+    $messages = $this->migration->getIdMap()->getMessages();
+    $count = 0;
+    foreach ($messages as $message) {
+      $count++;
+      $this->assertSame("The language 'tv' does not exist on this site.", $message->message);
+      $this->assertSame(MigrationInterface::MESSAGE_ERROR, (int) $message->level);
+    }
+    $this->assertSame(1, $count);
+  }
+
+  /**
+   * Tests language_default migration with unset default language variable.
+   */
+  public function testMigrationWithUnsetVariable(): void {
+    // Delete the language_default variable.
+    $this->sourceDatabase->delete('variable')
+      ->condition('name', 'language_default')
+      ->execute();
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    $messages = $this->migration->getIdMap()->getMessages()->fetchAll();
+    // Make sure there's no migration exceptions.
+    $this->assertEmpty($messages);
+    // Make sure the default langcode is 'en', as it was the default on D6 & D7.
+    $this->assertSame('en', $this->config('system.site')->get('default_langcode'));
+  }
+
+  /**
+   * Helper method to test the migration.
+   *
+   * @param string $langcode
+   *   The langcode of the default language.
+   */
+  protected function setDefaultLanguage($langcode) {
+    // The default language of the test fixture is English. Change it to
+    // something else before migrating, to be sure that the source site
+    // default language is migrated.
+    $value = 'O:8:"stdClass":11:{s:8:"language";s:2:"' . $langcode . '";s:4:"name";s:6:"French";s:6:"native";s:6:"French";s:9:"direction";s:1:"0";s:7:"enabled";i:1;s:7:"plurals";s:1:"0";s:7:"formula";s:0:"";s:6:"domain";s:0:"";s:6:"prefix";s:0:"";s:6:"weight";s:1:"0";s:10:"javascript";s:0:"";}';
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => $value])
+      ->condition('name', 'language_default')
+      ->execute();
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageContentCommentSettingsTest.php

@@ -0,0 +1,97 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests migration of language content comment settings.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageContentCommentSettingsTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'comment',
+    'language',
+    'content_translation',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installConfig(['comment']);
+    $this->executeMigrations([
+      'language',
+      'd6_comment_type',
+      'd6_language_content_comment_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of comment content language settings.
+   */
+  public function testLanguageCommentSettings(): void {
+    // Article and Employee content type have multilingual settings of 'Enabled,
+    // with Translation'. Assert that comments are not translatable and the
+    // default language is 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_article');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_article', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $third_party_settings = [
+      'content_translation' => [
+        'enabled' => FALSE,
+      ],
+    ];
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_employee');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_employee', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Sponsor content type has multilingual settings of 'Enabled'. Assert that
+    // comments are not translatable and the default language is
+    // 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_sponsor');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_sponsor', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Assert that non-translatable content is not translatable and the default
+    // language is 'site_default.
+    $not_translatable = [
+      'comment_node_company',
+      'comment_node_event',
+      'comment_node_page',
+      'comment_node_story',
+      'comment_node_test_event',
+      'comment_node_test_page',
+      'comment_node_test_planet',
+      'comment_node_test_story',
+      'comment_forum',
+      'comment_node_event',
+    ];
+    foreach ($not_translatable as $bundle) {
+      $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', $bundle);
+      $this->assertTrue($config->isDefaultConfiguration());
+      $this->assertFalse($config->isLanguageAlterable());
+      $this->assertSame('site_default', $config->getDefaultLangcode(), "Default language is not 'site_default' for comment bundle $bundle");
+    }
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageContentMenuSettingsTest.php

@@ -0,0 +1,73 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests migration of the ability to translate menu content.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageContentMenuSettingsTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'content_translation',
+    'menu_link_content',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    // Create some languages.
+    ConfigurableLanguage::createFromLangcode('en')->save();
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+    $this->executeMigrations([
+      'language',
+      'd6_language_content_menu_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of menu translation ability.
+   */
+  public function testLanguageMenuContent(): void {
+    $config = ContentLanguageSettings::load('menu_link_content.menu_link_content');
+    $this->assertInstanceOf(ContentLanguageSettings::class, $config);
+    $this->assertSame('menu_link_content', $config->getTargetEntityTypeId());
+    $this->assertSame('menu_link_content', $config->getTargetBundle());
+    $this->assertSame(LanguageInterface::LANGCODE_SITE_DEFAULT, $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+
+    // Test that menus are not alterable when the i18nmenu is not enabled.
+    $this->sourceDatabase->update('system')
+      ->fields(['status' => 0])
+      ->condition('name', 'i18nmenu')
+      ->execute();
+
+    /** @var \Drupal\migrate\Plugin\MigrationInterface $migration */
+    $migration = $this->getMigration('d6_language_content_menu_settings');
+    // Indicate we're rerunning a migration that's already run.
+    $migration->getIdMap()->prepareUpdate();
+    $this->executeMigration($migration);
+
+    $config = ContentLanguageSettings::load('menu_link_content.menu_link_content');
+    $this->assertInstanceOf(ContentLanguageSettings::class, $config);
+    $this->assertSame('menu_link_content', $config->getTargetEntityTypeId());
+    $this->assertSame('menu_link_content', $config->getTargetBundle());
+    $this->assertSame(LanguageInterface::LANGCODE_SITE_DEFAULT, $config->getDefaultLangcode());
+    $this->assertFalse($config->isLanguageAlterable());
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageContentSettingsTest.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests the migration of language-related settings.
+ *
+ * Settings tested include language content setting variables,
+ * language_content_type_$type, i18n_node_options_* and i18n_lock_node_*.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageContentSettingsTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'node',
+    'text',
+    'language',
+    'content_translation',
+    'menu_ui',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installConfig(['node']);
+    $this->installEntitySchema('node');
+    $this->executeMigrations([
+      'language',
+      'd6_node_type',
+      'd6_language_content_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of content language settings.
+   */
+  public function testLanguageContent(): void {
+    // Assert that a translatable content is still translatable.
+    $config = $this->config('language.content_settings.node.article');
+    $this->assertSame($config->get('target_entity_type_id'), 'node');
+    $this->assertSame($config->get('target_bundle'), 'article');
+    $this->assertSame($config->get('default_langcode'), 'current_interface');
+    $this->assertTrue($config->get('third_party_settings.content_translation.enabled'));
+
+    // Assert that a non-translatable content is not translatable.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'company');
+    $this->assertTrue($config->isDefaultConfiguration());
+    $this->assertFalse($config->isLanguageAlterable());
+    $this->assertSame($config->getDefaultLangcode(), 'site_default');
+
+    // Assert that a we can assign a language when there is no language lock.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'employee');
+    $this->assertSame($config->getDefaultLangcode(), 'current_interface');
+    $this->assertTrue($config->isLanguageAlterable());
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageContentTaxonomyVocabularySettingsTest.php

@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests migration of i18ntaxonomy vocabulary settings.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageContentTaxonomyVocabularySettingsTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'content_translation',
+    'taxonomy',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installEntitySchema('taxonomy_term');
+    $this->executeMigrations([
+      'language',
+      'd6_taxonomy_vocabulary',
+      'd6_language_content_taxonomy_vocabulary_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of 18ntaxonomy vocabulary settings.
+   */
+  public function testLanguageContentTaxonomy(): void {
+    $target_entity = 'taxonomy_term';
+    // Per Language.
+    $this->assertLanguageContentSettings($target_entity, 'vocabulary_1_i_0_', LanguageInterface::LANGCODE_SITE_DEFAULT, TRUE, ['enabled' => FALSE]);
+    // Set language to vocabulary.
+    $this->assertLanguageContentSettings($target_entity, 'vocabulary_2_i_1_', 'fr', FALSE, ['enabled' => FALSE]);
+    // Localize terms.
+    $this->assertLanguageContentSettings($target_entity, 'vocabulary_3_i_2_', LanguageInterface::LANGCODE_SITE_DEFAULT, TRUE, ['enabled' => FALSE]);
+    // None translation enabled.
+    $this->assertLanguageContentSettings($target_entity, 'vocabulary_name_much_longer_th', LanguageInterface::LANGCODE_SITE_DEFAULT, TRUE, ['enabled' => TRUE]);
+    $this->assertLanguageContentSettings($target_entity, 'tags', LanguageInterface::LANGCODE_SITE_DEFAULT, FALSE, ['enabled' => FALSE]);
+    $this->assertLanguageContentSettings($target_entity, 'forums', LanguageInterface::LANGCODE_SITE_DEFAULT, FALSE, ['enabled' => FALSE]);
+    $this->assertLanguageContentSettings($target_entity, 'type', LanguageInterface::LANGCODE_SITE_DEFAULT, FALSE, ['enabled' => FALSE]);
+  }
+
+  /**
+   * Asserts a content language settings configuration.
+   *
+   * @param string $target_entity
+   *   The expected target entity type.
+   * @param string $bundle
+   *   The expected bundle.
+   * @param string $default_langcode
+   *   The default language code.
+   * @param bool $language_alterable
+   *   The expected state of language alterable.
+   * @param array $third_party_settings
+   *   The content translation setting.
+   *
+   * @internal
+   */
+  public function assertLanguageContentSettings(string $target_entity, string $bundle, string $default_langcode, bool $language_alterable, array $third_party_settings): void {
+    $config = ContentLanguageSettings::load($target_entity . "." . $bundle);
+    $this->assertInstanceOf(ContentLanguageSettings::class, $config);
+    $this->assertSame($target_entity, $config->getTargetEntityTypeId());
+    $this->assertSame($bundle, $config->getTargetBundle());
+    $this->assertSame($default_langcode, $config->getDefaultLangcode());
+    $this->assertSame($language_alterable, $config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->getThirdPartySettings('content_translation'));
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageNegotiationSettingsTest.php

@@ -0,0 +1,169 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Tests the migration of language negotiation and language types.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageNegotiationSettingsTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Tests the migration with LANGUAGE_NEGOTIATION_PATH_DEFAULT.
+   */
+  public function testLanguageNegotiationWithDefaultPathPrefix(): void {
+    $this->executeMigrations([
+      'language',
+      'd6_language_negotiation_settings',
+      'language_prefixes_and_domains',
+      'd6_language_types',
+    ]);
+
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_PATH_PREFIX, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_prefixes = [
+      'en' => '',
+      'fr' => 'fr',
+      'zu' => 'zu',
+    ];
+    $this->assertSame($expected_prefixes, $config->get('url.prefixes'));
+
+    $config = $this->config('language.types');
+    $this->assertSame(['language_interface', 'language_content', 'language_url'], $config->get('all'));
+    $this->assertSame(['language_interface'], $config->get('configurable'));
+    $this->assertSame(['language-interface' => 0], $config->get('negotiation.language_content.enabled'));
+    $this->assertSame(['language-url' => 0, 'language-url-fallback' => 1], $config->get('negotiation.language_url.enabled'));
+    $expected_language_interface = [
+      'language-url' => 0,
+      'language-selected' => 1,
+    ];
+    $this->assertSame($expected_language_interface, $config->get('negotiation.language_interface.enabled'));
+  }
+
+  /**
+   * Tests the migration with LANGUAGE_NEGOTIATION_NONE.
+   */
+  public function testLanguageNegotiationWithNoNegotiation(): void {
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => serialize(0)])
+      ->condition('name', 'language_negotiation')
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd6_language_negotiation_settings',
+      'language_prefixes_and_domains',
+      'd6_language_types',
+    ]);
+
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_PATH_PREFIX, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+
+    $config = $this->config('language.types');
+    $this->assertSame(['language_interface', 'language_content', 'language_url'], $config->get('all'));
+    $this->assertSame(['language_interface'], $config->get('configurable'));
+    $this->assertSame(['language-interface' => 0], $config->get('negotiation.language_content.enabled'));
+    $this->assertSame(['language-url' => 0, 'language-url-fallback' => 1], $config->get('negotiation.language_url.enabled'));
+    $expected_language_interface = [
+      'language-selected' => 0,
+    ];
+    $this->assertSame($expected_language_interface, $config->get('negotiation.language_interface.enabled'));
+  }
+
+  /**
+   * Tests the migration with LANGUAGE_NEGOTIATION_PATH.
+   */
+  public function testLanguageNegotiationWithPathPrefix(): void {
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => serialize(2)])
+      ->condition('name', 'language_negotiation')
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd6_language_negotiation_settings',
+      'language_prefixes_and_domains',
+      'd6_language_types',
+    ]);
+
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_PATH_PREFIX, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_prefixes = [
+      'en' => '',
+      'fr' => 'fr',
+      'zu' => 'zu',
+    ];
+    $this->assertSame($expected_prefixes, $config->get('url.prefixes'));
+
+    $config = $this->config('language.types');
+    $this->assertSame(['language_interface', 'language_content', 'language_url'], $config->get('all'));
+    $this->assertSame(['language_interface'], $config->get('configurable'));
+    $this->assertSame(['language-interface' => 0], $config->get('negotiation.language_content.enabled'));
+    $this->assertSame(['language-url' => 0, 'language-url-fallback' => 1], $config->get('negotiation.language_url.enabled'));
+    $expected_language_interface = [
+      'language-url' => 0,
+      'language-user' => 1,
+      'language-browser' => 2,
+      'language-selected' => 3,
+    ];
+    $this->assertSame($expected_language_interface, $config->get('negotiation.language_interface.enabled'));
+  }
+
+  /**
+   * Tests the migration with LANGUAGE_NEGOTIATION_DOMAIN.
+   */
+  public function testLanguageNegotiationWithDomain(): void {
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => serialize(3)])
+      ->condition('name', 'language_negotiation')
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd6_language_negotiation_settings',
+      'language_prefixes_and_domains',
+      'd6_language_types',
+    ]);
+
+    global $base_url;
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_DOMAIN, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_domains = [
+      'en' => parse_url($base_url, PHP_URL_HOST),
+      'fr' => 'fr.drupal.org',
+      'zu' => 'zu.drupal.org',
+    ];
+    $this->assertSame($expected_domains, $config->get('url.domains'));
+
+    $config = $this->config('language.types');
+    $this->assertSame(['language_interface', 'language_content', 'language_url'], $config->get('all'));
+    $this->assertSame(['language_interface'], $config->get('configurable'));
+    $this->assertSame(['language-interface' => 0], $config->get('negotiation.language_content.enabled'));
+    $this->assertSame(['language-url' => 0, 'language-url-fallback' => 1], $config->get('negotiation.language_url.enabled'));
+    $expected_language_interface = [
+      'language-url' => 0,
+      'language-selected' => 1,
+    ];
+    $this->assertSame($expected_language_interface, $config->get('negotiation.language_interface.enabled'));
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageTest.php

@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d6;
+
+use Drupal\language\ConfigurableLanguageInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * @group migrate_drupal_6
+ */
+class MigrateLanguageTest extends MigrateDrupal6TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Asserts various properties of a configurable language entity.
+   *
+   * @param string $id
+   *   The language ID.
+   * @param string $label
+   *   The language name.
+   * @param string $direction
+   *   (optional) The language's direction (one of the DIRECTION_* constants in
+   *   ConfigurableLanguageInterface). Defaults to LTR.
+   * @param int $weight
+   *   (optional) The weight of the language. Defaults to 0.
+   *
+   * @internal
+   */
+  protected function assertLanguage(string $id, string $label, string $direction = ConfigurableLanguageInterface::DIRECTION_LTR, int $weight = 0): void {
+    /** @var \Drupal\language\ConfigurableLanguageInterface $language */
+    $language = ConfigurableLanguage::load($id);
+    $this->assertInstanceOf(ConfigurableLanguageInterface::class, $language);
+    $this->assertSame($label, $language->label());
+    $this->assertSame($direction, $language->getDirection());
+    $this->assertSame(0, $language->getWeight());
+    $this->assertFalse($language->isLocked());
+  }
+
+  /**
+   * Tests migration of Drupal 6 languages to configurable language entities.
+   */
+  public function testLanguageMigration(): void {
+    $this->executeMigration('language');
+    $this->assertLanguage('en', 'English');
+    $this->assertLanguage('fr', 'French');
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateDefaultLanguageTest.php

@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests the default language variable migration.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateDefaultLanguageTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Tests language_default migration with a non-existing language.
+   */
+  public function testMigrationWithExistingLanguage(): void {
+    $this->setDefaultLanguage('is');
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    // Tests the language is loaded and is the default language.
+    $default_language = ConfigurableLanguage::load('is');
+    $this->assertNotNull($default_language);
+    $this->assertSame('is', $this->config('system.site')->get('default_langcode'));
+  }
+
+  /**
+   * Tests language_default migration with a non-existing language.
+   */
+  public function testMigrationWithNonExistentLanguage(): void {
+    $this->setDefaultLanguage('tv');
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    // Tests the migration log contains an error message.
+    $messages = $this->migration->getIdMap()->getMessages();
+    $count = 0;
+    foreach ($messages as $message) {
+      $count++;
+      $this->assertSame("The language 'tv' does not exist on this site.", $message->message);
+      $this->assertSame(MigrationInterface::MESSAGE_ERROR, (int) $message->level);
+    }
+    $this->assertSame(1, $count);
+  }
+
+  /**
+   * Tests language_default migration with unset default language variable.
+   */
+  public function testMigrationWithUnsetVariable(): void {
+    // Delete the language_default variable.
+    $this->sourceDatabase->delete('variable')
+      ->condition('name', 'language_default')
+      ->execute();
+    $this->startCollectingMessages();
+    $this->executeMigrations(['language', 'default_language']);
+
+    $messages = $this->migration->getIdMap()->getMessages()->fetchAll();
+    // Make sure there's no migration exceptions.
+    $this->assertEmpty($messages);
+    // Make sure the default langcode is 'en', as it was the default on D6 & D7.
+    $this->assertSame('en', $this->config('system.site')->get('default_langcode'));
+  }
+
+  /**
+   * Helper method to test the migration.
+   *
+   * @param string $langcode
+   *   The langcode of the default language.
+   */
+  protected function setDefaultLanguage($langcode) {
+    // The default language of the test fixture is English. Change it to
+    // something else before migrating, to be sure that the source site
+    // default language is migrated.
+    $value = 'O:8:"stdClass":11:{s:8:"language";s:2:"' . $langcode . '";s:4:"name";s:6:"French";s:6:"native";s:6:"French";s:9:"direction";s:1:"0";s:7:"enabled";i:1;s:7:"plurals";s:1:"0";s:7:"formula";s:0:"";s:6:"domain";s:0:"";s:6:"prefix";s:0:"";s:6:"weight";s:1:"0";s:10:"javascript";s:0:"";}';
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => $value])
+      ->condition('name', 'language_default')
+      ->execute();
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageContentCommentSettingsNoEntityTranslationTest.php

@@ -0,0 +1,134 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\Core\Database\Database;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\migrate\Kernel\MigrateDumpAlterInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests language content comment settings migration with no entity translation.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageContentCommentSettingsNoEntityTranslationTest extends MigrateDrupal7TestBase implements MigrateDumpAlterInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'comment',
+    'content_translation',
+    'language',
+    'node',
+    'text',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->startCollectingMessages();
+    $this->migrateCommentTypes();
+    $this->executeMigrations([
+      'language',
+      'd7_language_content_comment_settings',
+    ]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function migrateDumpAlter(KernelTestBase $test) {
+    // Disable comment entity translation.
+    $db = Database::getConnection('default', 'migrate');
+
+    $db->update('variable')
+      ->condition('name', 'entity_translation_entity_types')
+      ->fields([
+        'value' => 'a:4:{s:4:"node";s:4:"node";s:13:"taxonomy_term";i:0;s:7:"comment";i:0;s:4:"user";i:0;}',
+      ])
+      ->execute();
+  }
+
+  /**
+   * Tests migration of content language settings.
+   */
+  public function testLanguageCommentSettings(): void {
+    // Confirm there is no message about a missing bundle.
+    $this->assertEmpty($this->migrateMessages, $this->migrateMessages['error'][0] ?? '');
+
+    // Article and Blog content type have multilingual settings of 'Enabled,
+    // with Translation'. Assert that comments are translatable and the default
+    // language is 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_article');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_article', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $third_party_settings = [
+      'content_translation' => [
+        'enabled' => FALSE,
+      ],
+    ];
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_blog');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_blog', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Page content type has multilingual settings of 'Enabled'. Assert that
+    // comments are translatable and default language is 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_page');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_page', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // The content type with a long name has multilingual settings of 'Enabled'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_a_thirty_two_char');
+    $this->assertFalse($config->isNew());
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_a_thirty_two_char', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Test content type has multilingual settings of 'Enabled, with field
+    // translation'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_test_content_type');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_test_content_type', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $third_party_settings = [
+      'content_translation' => [
+        'enabled' => FALSE,
+      ],
+    ];
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Assert that non-translatable content is not translatable and the default
+    // language is 'site_default.
+    $not_translatable = [
+      'comment_node_book',
+      'comment_forum',
+    ];
+    foreach ($not_translatable as $bundle) {
+      $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', $bundle);
+      $this->assertTrue($config->isDefaultConfiguration());
+      $this->assertFalse($config->isLanguageAlterable());
+      $this->assertSame('site_default', $config->getDefaultLangcode(), "Default language is not 'site_default' for comment bundle $bundle");
+    }
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageContentCommentSettingsTest.php

@@ -0,0 +1,116 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests migration of language comment settings.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageContentCommentSettingsTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'comment',
+    'content_translation',
+    'language',
+    'node',
+    'text',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->startCollectingMessages();
+    $this->migrateCommentTypes();
+    $this->executeMigrations([
+      'language',
+      'd7_language_content_comment_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of content language settings.
+   */
+  public function testLanguageCommentSettings(): void {
+    // Confirm there is no message about a missing bundle.
+    $this->assertEmpty($this->migrateMessages, $this->migrateMessages['error'][0] ?? '');
+
+    // Article and Blog content type have multilingual settings of 'Enabled,
+    // with Translation'. Assert that comments are translatable and the default
+    // language is 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_article');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_article', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $third_party_settings = [
+      'content_translation' => [
+        'enabled' => FALSE,
+      ],
+    ];
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_blog');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_blog', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Page content type has multilingual settings of 'Enabled'. Assert that
+    // comments are translatable and default language is 'current_interface'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_page');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_page', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // The content type with a long name has multilingual settings of 'Enabled'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_a_thirty_two_char');
+    $this->assertFalse($config->isNew());
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_a_thirty_two_char', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Test content type has multilingual settings of 'Enabled, with field
+    // translation'.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', 'comment_node_test_content_type');
+    $this->assertSame('comment', $config->getTargetEntityTypeId());
+    $this->assertSame('comment_node_test_content_type', $config->getTargetBundle());
+    $this->assertSame('current_interface', $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $third_party_settings = [
+      'content_translation' => [
+        'enabled' => TRUE,
+      ],
+    ];
+    $this->assertSame($third_party_settings, $config->get('third_party_settings'));
+
+    // Assert that non-translatable content is not translatable and the default
+    // language is 'site_default.
+    $not_translatable = [
+      'comment_node_book',
+      'comment_forum',
+    ];
+    foreach ($not_translatable as $bundle) {
+      $config = ContentLanguageSettings::loadByEntityTypeBundle('comment', $bundle);
+      $this->assertTrue($config->isDefaultConfiguration());
+      $this->assertFalse($config->isLanguageAlterable());
+      $this->assertSame('site_default', $config->getDefaultLangcode(), "Default language is not 'site_default' for comment bundle $bundle");
+    }
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageContentMenuSettingsTest.php

@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests migration of i18n_menu settings.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageContentMenuSettingsTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'content_translation',
+    'language',
+    'link',
+    'menu_link_content',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installEntitySchema('menu_link_content');
+    $this->executeMigrations([
+      'language',
+      'd7_menu',
+      'd7_language_content_menu_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of menu translation ability.
+   */
+  public function testLanguageContentMenu(): void {
+    $config = ContentLanguageSettings::load('menu_link_content.menu_link_content');
+    $this->assertInstanceOf(ContentLanguageSettings::class, $config);
+    $this->assertSame('menu_link_content', $config->getTargetEntityTypeId());
+    $this->assertSame('menu_link_content', $config->getTargetBundle());
+    $this->assertSame(LanguageInterface::LANGCODE_SITE_DEFAULT, $config->getDefaultLangcode());
+    $this->assertTrue($config->isLanguageAlterable());
+    $settings = [
+      'enabled' => TRUE,
+      'bundle_settings' => [
+        'untranslatable_fields_hide' => '0',
+      ],
+    ];
+    $this->assertSame($settings, $config->getThirdPartySettings('content_translation'));
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageContentSettingsTest.php

@@ -0,0 +1,87 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests the migration of language-related settings.
+ *
+ * Settings tested include the language content setting variables,
+ * language_content_type_$type, i18n_node_options_* and i18n_lock_node_*.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageContentSettingsTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'node',
+    'text',
+    'language',
+    'content_translation',
+    'menu_ui',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->migrateContentTypes();
+    $this->executeMigrations([
+      'language',
+      'd7_language_content_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of content language settings.
+   */
+  public function testLanguageContent(): void {
+    // Assert that a translatable content is still translatable.
+    $config = $this->config('language.content_settings.node.blog');
+    $this->assertSame($config->get('target_entity_type_id'), 'node');
+    $this->assertSame($config->get('target_bundle'), 'blog');
+    $this->assertSame($config->get('default_langcode'), 'current_interface');
+    $this->assertFalse($config->get('language_alterable'));
+    $this->assertTrue($config->get('third_party_settings.content_translation.enabled'));
+
+    // Assert that a translatable content is translatable.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'page');
+    $this->assertFalse($config->isDefaultConfiguration());
+    $this->assertTrue($config->isLanguageAlterable());
+    $this->assertSame($config->getDefaultLangcode(), 'current_interface');
+
+    // Assert that a non-translatable content is not translatable.
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'forum');
+    $this->assertTrue($config->isDefaultConfiguration());
+    $this->assertFalse($config->isLanguageAlterable());
+    $this->assertSame($config->getDefaultLangcode(), 'site_default');
+
+    // Make sure there's no migration exceptions.
+    $messages = $this->migration->getIdMap()->getMessages()->fetchAll();
+    $this->assertEmpty($messages);
+
+    // Assert that a content type translatable with entity_translation is still
+    // translatable.
+    $config = $this->config('language.content_settings.node.test_content_type');
+    $this->assertTrue($config->get('third_party_settings.content_translation.enabled'));
+    $this->assertSame($config->get('default_langcode'), 'und');
+
+    // Assert that a content type without a 'language_content_type' variable is
+    // not translatable
+    $config = ContentLanguageSettings::loadByEntityTypeBundle('node', 'book');
+    $this->assertTrue($config->isDefaultConfiguration());
+    $this->assertFalse($config->isLanguageAlterable());
+    $this->assertSame($config->getDefaultLangcode(), 'site_default');
+
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageContentTaxonomyVocabularySettingsTest.php

@@ -0,0 +1,85 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests migration of i18ntaxonomy vocabulary settings.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageContentTaxonomyVocabularySettingsTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'content_translation',
+    'taxonomy',
+    'text',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->installEntitySchema('taxonomy_term');
+    $this->executeMigrations([
+      'language',
+      'd7_taxonomy_vocabulary',
+      'd7_language_content_taxonomy_vocabulary_settings',
+    ]);
+  }
+
+  /**
+   * Tests migration of 18ntaxonomy vocabulary settings.
+   */
+  public function testLanguageContentTaxonomy(): void {
+    $target_entity = 'taxonomy_term';
+    // No multilingual options for terms, i18n_mode = 0.
+    $this->assertLanguageContentSettings($target_entity, 'tags', LanguageInterface::LANGCODE_NOT_SPECIFIED, FALSE, ['enabled' => FALSE]);
+    $this->assertLanguageContentSettings($target_entity, 'sujet_de_discussion', LanguageInterface::LANGCODE_NOT_SPECIFIED, FALSE, ['enabled' => FALSE]);
+    $this->assertLanguageContentSettings($target_entity, 'vocabulary_name_much_longer_th', LanguageInterface::LANGCODE_NOT_SPECIFIED, FALSE, ['enabled' => FALSE]);
+    $this->assertLanguageContentSettings($target_entity, 'test_vocabulary', LanguageInterface::LANGCODE_NOT_SPECIFIED, FALSE, ['enabled' => FALSE]);
+    // Localize, i18n_mode = 1.
+    $this->assertLanguageContentSettings($target_entity, 'vocablocalized', LanguageInterface::LANGCODE_NOT_SPECIFIED, TRUE, ['enabled' => TRUE]);
+    // Translate, i18n_mode = 4.
+    $this->assertLanguageContentSettings($target_entity, 'vocabtranslate', LanguageInterface::LANGCODE_NOT_SPECIFIED, TRUE, ['enabled' => FALSE]);
+    // Fixed language, i18n_mode = 2.
+    $this->assertLanguageContentSettings($target_entity, 'vocabfixed', 'fr', FALSE, ['enabled' => FALSE]);
+  }
+
+  /**
+   * Asserts a content language settings configuration.
+   *
+   * @param string $target_entity
+   *   The expected target entity type.
+   * @param string $bundle
+   *   The expected bundle.
+   * @param string $default_langcode
+   *   The default language code.
+   * @param bool $language_alterable
+   *   The expected state of language alterable.
+   * @param array $third_party_settings
+   *   The content translation setting.
+   *
+   * @internal
+   */
+  public function assertLanguageContentSettings(string $target_entity, string $bundle, string $default_langcode, bool $language_alterable, array $third_party_settings): void {
+    $config = ContentLanguageSettings::load($target_entity . '.' . $bundle);
+    $this->assertInstanceOf(ContentLanguageSettings::class, $config);
+    $this->assertSame($target_entity, $config->getTargetEntityTypeId());
+    $this->assertSame($bundle, $config->getTargetBundle());
+    $this->assertSame($default_langcode, $config->getDefaultLangcode());
+    $this->assertSame($language_alterable, $config->isLanguageAlterable());
+    $this->assertSame($third_party_settings, $config->getThirdPartySettings('content_translation'));
+  }
+
+}

core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageNegotiationSettingsTest.php

@@ -0,0 +1,144 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Migrate\d7;
+
+use Drupal\language\Plugin\LanguageNegotiation\LanguageNegotiationUrl;
+use Drupal\Tests\migrate_drupal\Kernel\d7\MigrateDrupal7TestBase;
+
+/**
+ * Tests the migration of language negotiation.
+ *
+ * @group migrate_drupal_7
+ */
+class MigrateLanguageNegotiationSettingsTest extends MigrateDrupal7TestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language'];
+
+  /**
+   * Tests migration of language types variables to language.types.yml.
+   */
+  public function testLanguageTypes(): void {
+    $this->executeMigrations([
+      'language',
+      'd7_language_negotiation_settings',
+      'd7_language_types',
+    ]);
+
+    $config = $this->config('language.types');
+    $this->assertSame(['language_content', 'language_url', 'language_interface'], $config->get('all'));
+    $this->assertSame(['language_content', 'language_interface'], $config->get('configurable'));
+    $this->assertSame(['enabled' => ['language-interface' => 0]], $config->get('negotiation.language_content'));
+    $this->assertSame(['enabled' => ['language-url' => 0, 'language-url-fallback' => 1]], $config->get('negotiation.language_url'));
+    $expected_language_interface = [
+      'enabled' => [
+        'language-url' => -9,
+        'language-user' => -10,
+        'language-selected' => -6,
+      ],
+      'method_weights' => [
+        'language-url' => -9,
+        'language-session' => -8,
+        'language-user' => -10,
+        'language-browser' => -7,
+        'language-selected' => -6,
+      ],
+    ];
+    $this->assertSame($expected_language_interface, $config->get('negotiation.language_interface'));
+  }
+
+  /**
+   * Tests the migration with prefix negotiation.
+   */
+  public function testLanguageNegotiationWithPrefix(): void {
+    $this->sourceDatabase->update('languages')
+      ->fields(['domain' => ''])
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd7_language_negotiation_settings',
+      'language_prefixes_and_domains',
+    ]);
+
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_PATH_PREFIX, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_prefixes = [
+      'en' => '',
+      'fr' => 'fr',
+      'is' => 'is',
+    ];
+    $this->assertSame($expected_prefixes, $config->get('url.prefixes'));
+
+    // If prefix negotiation is used, make sure that no domains are migrated.
+    // Otherwise there will be validation errors when trying to save URL
+    // language detection configuration from the UI.
+    $expected_domains = [
+      'en' => '',
+      'fr' => '',
+      'is' => '',
+    ];
+    $this->assertSame($expected_domains, $config->get('url.domains'));
+  }
+
+  /**
+   * Tests the migration with domain negotiation.
+   */
+  public function testLanguageNegotiationWithDomain(): void {
+    $this->sourceDatabase->update('variable')
+      ->fields(['value' => serialize(1)])
+      ->condition('name', 'locale_language_negotiation_url_part')
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd7_language_negotiation_settings',
+      'language_prefixes_and_domains',
+    ]);
+
+    global $base_url;
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_DOMAIN, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_domains = [
+      'en' => parse_url($base_url, PHP_URL_HOST),
+      'fr' => 'fr.drupal.org',
+      'is' => 'is.drupal.org',
+    ];
+    $this->assertSame($expected_domains, $config->get('url.domains'));
+  }
+
+  /**
+   * Tests the migration with non-existent variables.
+   */
+  public function testLanguageNegotiationWithNonExistentVariables(): void {
+    $this->sourceDatabase->delete('variable')
+      ->condition('name', ['local_language_negotiation_url_part', 'local_language_negotiation_session_param'], 'IN')
+      ->execute();
+
+    $this->executeMigrations([
+      'language',
+      'd6_language_negotiation_settings',
+      'language_prefixes_and_domains',
+    ]);
+
+    $config = $this->config('language.negotiation');
+    $this->assertSame('language', $config->get('session.parameter'));
+    $this->assertSame(LanguageNegotiationUrl::CONFIG_PATH_PREFIX, $config->get('url.source'));
+    $this->assertSame('site_default', $config->get('selected_langcode'));
+    $expected_prefixes = [
+      'en' => '',
+      'fr' => 'fr',
+      'is' => 'is',
+    ];
+    $this->assertSame($expected_prefixes, $config->get('url.prefixes'));
+  }
+
+}

core/modules/language/tests/src/Kernel/OverriddenConfigImportTest.php

@@ -0,0 +1,102 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel;
+
+use Drupal\Core\Config\ConfigImporter;
+use Drupal\Core\Config\StorageComparer;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests importing of config with language overrides.
+ *
+ * @group language
+ */
+class OverriddenConfigImportTest extends KernelTestBase {
+
+  /**
+   * Config Importer object used for testing.
+   *
+   * @var \Drupal\Core\Config\ConfigImporter
+   */
+  protected $configImporter;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['system', 'language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installConfig(['system']);
+    $this->copyConfig($this->container->get('config.storage'), $this->container->get('config.storage.sync'));
+
+    // Set up the ConfigImporter object for testing.
+    $storage_comparer = new StorageComparer(
+      $this->container->get('config.storage.sync'),
+      $this->container->get('config.storage')
+    );
+    $this->configImporter = new ConfigImporter(
+      $storage_comparer->createChangelist(),
+      $this->container->get('event_dispatcher'),
+      $this->container->get('config.manager'),
+      $this->container->get('lock'),
+      $this->container->get('config.typed'),
+      $this->container->get('module_handler'),
+      $this->container->get('module_installer'),
+      $this->container->get('theme_handler'),
+      $this->container->get('string_translation'),
+      $this->container->get('extension.list.module'),
+      $this->container->get('extension.list.theme')
+    );
+  }
+
+  /**
+   * Tests importing overridden config alongside config in the default language.
+   */
+  public function testConfigImportUpdates(): void {
+    $storage = $this->container->get('config.storage');
+    $sync = $this->container->get('config.storage.sync');
+    /** @var \Drupal\language\ConfigurableLanguageManagerInterface $language_manager */
+    $language_manager = $this->container->get('language_manager');
+
+    // Make a change to the site configuration in the default collection.
+    $data = $storage->read('system.site');
+    $data['name'] = 'English site name';
+    $sync->write('system.site', $data);
+
+    // Also make a change to the same config object, but using a language
+    // override.
+    /** @var \Drupal\Core\Config\StorageInterface $overridden_sync */
+    $overridden_sync = $sync->createCollection('language.fr');
+    $overridden_sync->write('system.site', ['name' => 'French site name']);
+
+    // Before we start the import, the change to the site name should not be
+    // present. This action also primes the cache in the config factory so that
+    // we can test whether the cached data is correctly updated.
+    $config = $this->config('system.site');
+    $this->assertNotEquals('English site name', $config->getRawData()['name']);
+
+    // Before the import is started the site name should not yet be overridden.
+    $this->assertFalse($config->hasOverrides());
+    $override = $language_manager->getLanguageConfigOverride('fr', 'system.site');
+    $this->assertTrue($override->isNew());
+
+    // Start the import of the new configuration.
+    $this->configImporter->reset()->import();
+
+    // Verify the new site name in the default language.
+    $config = $this->config('system.site')->getRawData();
+    $this->assertEquals('English site name', $config['name']);
+
+    // Verify the overridden site name.
+    $override = $language_manager->getLanguageConfigOverride('fr', 'system.site');
+    $this->assertEquals('French site name', $override->get('name'));
+  }
+
+}

core/modules/language/tests/src/Kernel/Plugin/migrate/source/LanguageTest.php

@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Plugin\migrate\source;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests the language source plugin.
+ *
+ * @covers \Drupal\language\Plugin\migrate\source\Language
+ * @group language
+ */
+class LanguageTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['languages'] = [
+      [
+        'language' => 'en',
+        'name' => 'English',
+        'native' => 'English',
+        'direction' => '0',
+        'enabled' => '1',
+        'plurals' => '0',
+        'formula' => '',
+        'domain' => '',
+        'prefix' => '',
+        'weight' => '0',
+        'javascript' => '',
+      ],
+      [
+        'language' => 'fr',
+        'name' => 'French',
+        'native' => 'Français',
+        'direction' => '0',
+        'enabled' => '0',
+        'plurals' => '2',
+        'formula' => '($n>1)',
+        'domain' => '',
+        'prefix' => 'fr',
+        'weight' => '0',
+        'javascript' => '',
+      ],
+    ];
+
+    // The expected results.
+    $tests[0]['expected_data'] = [
+      [
+        'language' => 'en',
+        'name' => 'English',
+        'native' => 'English',
+        'direction' => '0',
+        'enabled' => '1',
+        'plurals' => '0',
+        'formula' => '',
+        'domain' => '',
+        'prefix' => '',
+        'weight' => '0',
+        'javascript' => '',
+      ],
+      [
+        'language' => 'fr',
+        'name' => 'French',
+        'native' => 'Français',
+        'direction' => '0',
+        'enabled' => '0',
+        'plurals' => '2',
+        'formula' => '($n>1)',
+        'domain' => '',
+        'prefix' => 'fr',
+        'weight' => '0',
+        'javascript' => '',
+      ],
+    ];
+
+    return $tests;
+  }
+
+}

core/modules/language/tests/src/Kernel/Plugin/migrate/source/d6/LanguageContentSettingsTest.php

@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests menu source plugin.
+ *
+ * @covers \Drupal\language\Plugin\migrate\source\d6\LanguageContentSettings
+ *
+ * @group language
+ */
+class LanguageContentSettingsTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['node_type'] = [
+      [
+        'type' => 'article',
+        'name' => 'Article',
+        'module' => 'node',
+        'description' => 'An <em>article</em>, content type.',
+        'help' => '',
+        'has_title' => 1,
+        'title_label' => 'Title',
+        'has_body' => 1,
+        'body_label' => 'Body',
+        'min_word_count' => 0,
+        'custom' => 1,
+        'modified' => 1,
+        'locked' => 0,
+        'orig_type' => 'story',
+      ],
+      [
+        'type' => 'company',
+        'name' => 'Company',
+        'module' => 'node',
+        'description' => 'Company node type',
+        'help' => '',
+        'has_title' => 1,
+        'title_label' => 'Name',
+        'has_body' => 1,
+        'body_label' => 'Description',
+        'min_word_count' => 0,
+        'custom' => 0,
+        'modified' => 1,
+        'locked' => 0,
+        'orig_type' => 'company',
+      ],
+    ];
+
+    foreach ($tests[0]['source_data']['node_type'] as $node_type) {
+      $tests[0]['expected_data'][] = [
+        'type' => $node_type['type'],
+        'language_content_type' => NULL,
+        'i18n_lock_node' => 0,
+      ];
+    }
+
+    return $tests;
+  }
+
+}

core/modules/language/tests/src/Kernel/Plugin/migrate/source/d6/LanguageContentTaxonomyVocabularySettingsTest.php

@@ -0,0 +1,109 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Plugin\migrate\source\d6;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests i18ntaxonomy vocabulary setting source plugin.
+ *
+ * @covers \Drupal\language\Plugin\migrate\source\d6\LanguageContentSettingsTaxonomyVocabulary
+ *
+ * @group language
+ */
+class LanguageContentTaxonomyVocabularySettingsTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['taxonomy', 'language', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['vocabulary'] = [
+      [
+        'vid' => 1,
+        'name' => 'Tags',
+        'description' => 'Tags description.',
+        'help' => 1,
+        'relations' => 0,
+        'hierarchy' => 0,
+        'multiple' => 0,
+        'required' => 0,
+        'tags' => 1,
+        'module' => 'taxonomy',
+        'weight' => 0,
+        'language' => '',
+      ],
+      [
+        'vid' => 2,
+        'name' => 'Categories',
+        'description' => 'Categories description.',
+        'help' => 1,
+        'relations' => 1,
+        'hierarchy' => 1,
+        'multiple' => 0,
+        'required' => 1,
+        'tags' => 0,
+        'module' => 'taxonomy',
+        'weight' => 0,
+        'language' => 'zu',
+      ],
+    ];
+    $tests[0]['source_data']['variable'] = [
+      [
+        'name' => 'i18ntaxonomy_vocabulary',
+        'value' => 'a:4:{i:1;s:1:"3";i:2;s:1:"2";i:3;s:1:"3";i:5;s:1:"1";}',
+      ],
+    ];
+
+    $tests[0]['expected_data'] = [
+      [
+        'vid' => 1,
+        'language' => '',
+        'state' => 3,
+      ],
+      [
+        'vid' => 2,
+        'language' => 'zu',
+        'state' => 2,
+      ],
+    ];
+
+    // Test without a language column in the database.
+    $tests[1] = $tests[0];
+    foreach ($tests[1]['source_data']['vocabulary'] as $key => $row) {
+      unset($tests[1]['source_data']['vocabulary'][$key]['language']);
+    }
+    $tests[1]['source_data']['variable'] = [
+      [
+        'name' => 'i18ntaxonomy_vocabulary',
+        'value' => 'a:4:{i:1;s:1:"0";i:2;s:1:"0";i:3;s:1:"3";i:5;s:1:"1";}',
+      ],
+    ];
+    $tests[1]['expected_data'] = [
+      [
+        'vid' => 1,
+        'state' => 0,
+      ],
+      [
+        'vid' => 2,
+        'state' => 0,
+      ],
+    ];
+
+    // Test without a i18ntaxonomy_vocabulary variable.
+    $tests[2] = $tests[1];
+    unset($tests[2]['source_data']['variable']);
+    return $tests;
+
+  }
+
+}

core/modules/language/tests/src/Kernel/Plugin/migrate/source/d7/LanguageContentSettingsTest.php

@@ -0,0 +1,74 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Plugin\migrate\source\d7;
+
+use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+
+/**
+ * Tests menu source plugin.
+ *
+ * @covers \Drupal\language\Plugin\migrate\source\d7\LanguageContentSettings
+ *
+ * @group language
+ */
+class LanguageContentSettingsTest extends MigrateSqlSourceTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function providerSource() {
+    $tests = [];
+
+    // The source data.
+    $tests[0]['source_data']['node_type'] = [
+      [
+        'type' => 'article',
+        'name' => 'Article',
+        'base' => 'node_content',
+        'module' => 'node',
+        'description' => 'Use <em>articles</em> for time-sensitive content like news, press releases or blog posts.',
+        'help' => 'Help text for articles',
+        'has_title' => 1,
+        'title_label' => 'Title',
+        'custom' => 1,
+        'modified' => 1,
+        'locked' => 0,
+        'disabled' => 0,
+        'orig_type' => 'article',
+      ],
+      [
+        'type' => 'blog',
+        'name' => 'Blog entry',
+        'base' => 'blog',
+        'module' => 'blog',
+        'description' => 'Use for multi-user blogs. Every user gets a personal blog.',
+        'help' => 'Blog away, good sir!',
+        'has_title' => 1,
+        'title_label' => 'Title',
+        'custom' => 0,
+        'modified' => 1,
+        'locked' => 1,
+        'disabled' => 0,
+        'orig_type' => 'blog',
+      ],
+    ];
+
+    foreach ($tests[0]['source_data']['node_type'] as $node_type) {
+      $tests[0]['expected_data'][] = [
+        'type' => $node_type['type'],
+        'language_content_type' => NULL,
+        'i18n_lock_node' => 0,
+      ];
+    }
+
+    return $tests;
+  }
+
+}

core/modules/language/tests/src/Kernel/Plugin/migrate/source/d7/LanguageContentTaxonomyVocabularySettingsTest.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Plugin\migrate\source\d7;
+
+use Drupal\Tests\taxonomy\Kernel\Plugin\migrate\source\d7\VocabularyTest;
+
+/**
+ * Tests i18ntaxonomy vocabulary setting source plugin.
+ *
+ * @covers \Drupal\language\Plugin\migrate\source\d7\LanguageContentSettingsTaxonomyVocabulary
+ *
+ * @group language
+ */
+class LanguageContentTaxonomyVocabularySettingsTest extends VocabularyTest {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['taxonomy', 'language', 'migrate_drupal'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function providerSource() {
+    // Get the source data from parent.
+    $tests = parent::providerSource();
+
+    foreach ($tests as &$test) {
+      // Add the extra columns provided by i18n_taxonomy.
+      foreach ($test['source_data']['taxonomy_vocabulary'] as &$vocabulary) {
+        $vocabulary['language'] = 'und';
+        $vocabulary['i18n_mode'] = 2;
+      }
+      foreach ($test['expected_data'] as &$expected) {
+        $expected['language'] = 'und';
+        $expected['i18n_mode'] = 2;
+      }
+    }
+    return $tests;
+  }
+
+}

core/modules/language/tests/src/Kernel/Views/ArgumentLanguageTest.php

@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Views;
+
+use Drupal\views\Views;
+
+/**
+ * Tests the argument language handler.
+ *
+ * @group language
+ * @see \Drupal\language\Plugin\views\argument\Language.php
+ */
+class ArgumentLanguageTest extends LanguageTestBase {
+
+  /**
+   * Views used by this test.
+   *
+   * @var array
+   */
+  public static $testViews = ['test_view'];
+
+  /**
+   * Tests the language argument.
+   */
+  public function testArgument(): void {
+    $view = Views::getView('test_view');
+    foreach (['en' => 'John', 'xx-lolspeak' => 'George'] as $langcode => $name) {
+      $view->setDisplay();
+      $view->displayHandlers->get('default')->overrideOption('arguments', [
+        'langcode' => [
+          'id' => 'langcode',
+          'table' => 'views_test_data',
+          'field' => 'langcode',
+        ],
+      ]);
+      $this->executeView($view, [$langcode]);
+
+      $expected = [
+        ['name' => $name],
+      ];
+      $this->assertIdenticalResultset($view, $expected, ['views_test_data_name' => 'name']);
+      $view->destroy();
+    }
+  }
+
+}

core/modules/language/tests/src/Kernel/Views/FieldLanguageTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Views;
+
+use Drupal\views\Views;
+
+/**
+ * Tests the field language handler.
+ *
+ * @group language
+ * @see \Drupal\language\Plugin\views\field\Language
+ */
+class FieldLanguageTest extends LanguageTestBase {
+
+  /**
+   * Views used by this test.
+   *
+   * @var array
+   */
+  public static $testViews = ['test_view'];
+
+  /**
+   * Tests the language field.
+   */
+  public function testField(): void {
+    $view = Views::getView('test_view');
+    $view->setDisplay();
+    $view->displayHandlers->get('default')->overrideOption('fields', [
+      'langcode' => [
+        'id' => 'langcode',
+        'table' => 'views_test_data',
+        'field' => 'langcode',
+      ],
+    ]);
+    $this->executeView($view);
+
+    $this->assertEquals('English', $view->field['langcode']->advancedRender($view->result[0]));
+    $this->assertEquals('Lolspeak', $view->field['langcode']->advancedRender($view->result[1]));
+  }
+
+}

core/modules/language/tests/src/Kernel/Views/FilterLanguageTest.php

@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Views;
+
+use Drupal\views\Views;
+
+/**
+ * Tests the filter language handler.
+ *
+ * @group language
+ * @see \Drupal\language\Plugin\views\filter\Language
+ */
+class FilterLanguageTest extends LanguageTestBase {
+
+  /**
+   * Views used by this test.
+   *
+   * @var array
+   */
+  public static $testViews = ['test_view'];
+
+  /**
+   * Tests the language filter.
+   */
+  public function testFilter(): void {
+    $view = Views::getView('test_view');
+    foreach (['en' => 'John', 'xx-lolspeak' => 'George'] as $langcode => $name) {
+      $view->setDisplay();
+      $view->displayHandlers->get('default')->overrideOption('filters', [
+        'langcode' => [
+          'id' => 'langcode',
+          'table' => 'views_test_data',
+          'field' => 'langcode',
+          'value' => [$langcode => $langcode],
+        ],
+      ]);
+      $this->executeView($view);
+
+      $expected = [
+        ['name' => $name],
+      ];
+      $this->assertIdenticalResultset($view, $expected, ['views_test_data_name' => 'name']);
+
+      $expected = [
+        '***LANGUAGE_site_default***',
+        '***LANGUAGE_language_interface***',
+        'en',
+        'xx-lolspeak',
+        'und',
+        'zxx',
+      ];
+      $this->assertSame($expected, array_keys($view->filter['langcode']->getValueOptions()));
+
+      $view->destroy();
+    }
+  }
+
+}

core/modules/language/tests/src/Kernel/Views/LanguageTestBase.php

@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Kernel\Views;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\views\Kernel\ViewsKernelTestBase;
+
+/**
+ * Defines the base class for all Language handler tests.
+ */
+abstract class LanguageTestBase extends ViewsKernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['system', 'language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp($import_test_views = TRUE): void {
+    parent::setUp();
+    $this->installConfig(['language']);
+
+    // Create another language beside English.
+    ConfigurableLanguage::create(['id' => 'xx-lolspeak', 'label' => 'Lolspeak'])->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function schemaDefinition() {
+    $schema = parent::schemaDefinition();
+    $schema['views_test_data']['fields']['langcode'] = [
+      'description' => 'The {language}.langcode of this beatle.',
+      'type' => 'varchar',
+      'length' => 12,
+      'default' => '',
+    ];
+
+    return $schema;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function viewsData() {
+    $data = parent::viewsData();
+    $data['views_test_data']['langcode'] = [
+      'title' => t('Langcode'),
+      'help' => t('Langcode'),
+      'field' => [
+        'id' => 'language',
+      ],
+      'argument' => [
+        'id' => 'language',
+      ],
+      'filter' => [
+        'id' => 'language',
+      ],
+    ];
+
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function dataSet() {
+    $data = parent::dataSet();
+    $data[0]['langcode'] = 'en';
+    $data[1]['langcode'] = 'xx-lolspeak';
+    $data[2]['langcode'] = '';
+    $data[3]['langcode'] = '';
+    $data[4]['langcode'] = '';
+
+    return $data;
+  }
+
+}

core/modules/language/tests/src/Traits/LanguageTestTrait.php

@@ -0,0 +1,92 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Traits;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\language\ConfigurableLanguageInterface;
+use Drupal\language\ContentLanguageSettingsInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\language\Entity\ContentLanguageSettings;
+
+/**
+ * Provides an API to programmatically manage languages in tests.
+ */
+trait LanguageTestTrait {
+
+  /**
+   * Creates a configurable language object from a langcode.
+   *
+   * @param string $langcode
+   *   The language code to use to create the object.
+   *
+   * @return \Drupal\Core\Language\ConfigurableLanguageInterface
+   *   The created language.
+   *
+   * @see \Drupal\Core\Language\LanguageManager::getStandardLanguageList()
+   */
+  public static function createLanguageFromLangcode(string $langcode): ConfigurableLanguageInterface {
+    $configurable_language = ConfigurableLanguage::createFromLangcode($langcode);
+    $configurable_language->save();
+    return $configurable_language;
+  }
+
+  /**
+   * Enables translations for the given entity type bundle.
+   *
+   * @param string $entity_type_id
+   *   The ID of the entity type.
+   * @param string $bundle
+   *   The bundle name.
+   * @param string|null $default_langcode
+   *   The language code to use as the default language.
+   *
+   * @return \Drupal\language\ContentLanguageSettingsInterface
+   *   The saved content language config entity.
+   */
+  public static function enableBundleTranslation(string $entity_type_id, string $bundle, ?string $default_langcode = LanguageInterface::LANGCODE_SITE_DEFAULT): ContentLanguageSettingsInterface {
+    $content_language_settings = ContentLanguageSettings::loadByEntityTypeBundle($entity_type_id, $bundle)
+      ->setDefaultLangcode($default_langcode)
+      ->setLanguageAlterable(TRUE);
+    $content_language_settings->save();
+    return $content_language_settings;
+  }
+
+  /**
+   * Disables translations for the given entity type bundle.
+   *
+   * @param string $entity_type_id
+   *   The ID of the entity type.
+   * @param string $bundle
+   *   The bundle name.
+   */
+  public static function disableBundleTranslation(string $entity_type_id, string $bundle) {
+    // @todo Move to API call when it exists, to be added at
+    // https://www.drupal.org/project/drupal/issues/3408046
+    $content_language_settings = ContentLanguageSettings::loadByEntityTypeBundle($entity_type_id, $bundle);
+    $content_language_settings->setLanguageAlterable(FALSE)
+      ->save();
+    $content_language_settings->delete();
+  }
+
+  /**
+   * Sets and saves a given field instance translation status.
+   *
+   * @param string $entity_type_id
+   *   The ID of the entity type.
+   * @param string $bundle
+   *   The bundle name.
+   * @param string $field_name
+   *   The name of the field.
+   * @param bool $status
+   *   Whether the field should be translatable or not.
+   */
+  public static function setFieldTranslatable(string $entity_type_id, string $bundle, string $field_name, bool $status): void {
+    FieldConfig::loadByName($entity_type_id, $bundle, $field_name)
+      ->setTranslatable($status)
+      ->save();
+  }
+
+}

core/modules/language/tests/src/Unit/Config/LanguageConfigOverrideTest.php

@@ -0,0 +1,103 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Unit\Config;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\language\Config\LanguageConfigOverride;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\language\Config\LanguageConfigOverride
+ * @group Config
+ * @group language
+ */
+class LanguageConfigOverrideTest extends UnitTestCase {
+
+  /**
+   * Language configuration override.
+   *
+   * @var \Drupal\language\Config\LanguageConfigOverride
+   */
+  protected $configTranslation;
+
+  /**
+   * Storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $storage;
+
+  /**
+   * Event Dispatcher.
+   *
+   * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $eventDispatcher;
+
+  /**
+   * Typed Config.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $typedConfig;
+
+  /**
+   * The mocked cache tags invalidator.
+   *
+   * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $cacheTagsInvalidator;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->storage = $this->createMock('Drupal\Core\Config\StorageInterface');
+    $this->eventDispatcher = $this->createMock('Symfony\Contracts\EventDispatcher\EventDispatcherInterface');
+    $this->typedConfig = $this->createMock('\Drupal\Core\Config\TypedConfigManagerInterface');
+    $this->configTranslation = new LanguageConfigOverride('config.test', $this->storage, $this->typedConfig, $this->eventDispatcher);
+    $this->cacheTagsInvalidator = $this->createMock('Drupal\Core\Cache\CacheTagsInvalidatorInterface');
+
+    $container = new ContainerBuilder();
+    $container->set('cache_tags.invalidator', $this->cacheTagsInvalidator);
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * @covers ::save
+   */
+  public function testSaveNew(): void {
+    $this->cacheTagsInvalidator->expects($this->once())
+      ->method('invalidateTags')
+      ->with(['config:config.test']);
+    $this->assertTrue($this->configTranslation->isNew());
+    $this->configTranslation->save();
+  }
+
+  /**
+   * @covers ::save
+   */
+  public function testSaveExisting(): void {
+    $this->cacheTagsInvalidator->expects($this->once())
+      ->method('invalidateTags')
+      ->with(['config:config.test']);
+    $this->configTranslation->initWithData([]);
+    $this->configTranslation->save();
+  }
+
+  /**
+   * @covers ::delete
+   */
+  public function testDelete(): void {
+    $this->cacheTagsInvalidator->expects($this->once())
+      ->method('invalidateTags')
+      ->with(['config:config.test']);
+    $this->configTranslation->initWithData([]);
+    $this->configTranslation->delete();
+  }
+
+}

core/modules/language/tests/src/Unit/ConfigurableLanguageUnitTest.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Unit;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the ConfigurableLanguage entity class.
+ *
+ * @group language
+ * @coversDefaultClass \Drupal\language\Entity\ConfigurableLanguage
+ * @see \Drupal\language\Entity\ConfigurableLanguage.
+ */
+class ConfigurableLanguageUnitTest extends UnitTestCase {
+
+  /**
+   * @covers ::getDirection
+   */
+  public function testDirection(): void {
+    // Direction of language writing, an integer. Usually either
+    // ConfigurableLanguage::DIRECTION_LTR or
+    // ConfigurableLanguage::DIRECTION_RTL.
+    $configurableLanguage = new ConfigurableLanguage(['direction' => ConfigurableLanguage::DIRECTION_LTR], 'configurable_language');
+    $this->assertEquals(ConfigurableLanguage::DIRECTION_LTR, $configurableLanguage->getDirection());
+
+    // Test direction again, setting direction to RTL.
+    $configurableLanguage = new ConfigurableLanguage(['direction' => ConfigurableLanguage::DIRECTION_RTL], 'configurable_language');
+    $this->assertEquals(ConfigurableLanguage::DIRECTION_RTL, $configurableLanguage->getDirection());
+  }
+
+  /**
+   * @covers ::getWeight
+   * @covers ::setWeight
+   */
+  public function testWeight(): void {
+    // The weight, an integer. Used to order languages with larger positive
+    // weights sinking items toward the bottom of lists.
+    $configurableLanguage = new ConfigurableLanguage(['weight' => -5], 'configurable_language');
+    $this->assertEquals(-5, $configurableLanguage->getWeight());
+    $this->assertEquals(13, $configurableLanguage->setWeight(13)->getWeight());
+  }
+
+}

core/modules/language/tests/src/Unit/ContentLanguageSettingsUnitTest.php

@@ -0,0 +1,311 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\language\Unit;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\language\Entity\ContentLanguageSettings;
+use Drupal\Tests\UnitTestCase;
+use Drupal\TestTools\Random;
+
+/**
+ * @coversDefaultClass \Drupal\language\Entity\ContentLanguageSettings
+ * @group language
+ */
+class ContentLanguageSettingsUnitTest extends UnitTestCase {
+
+  /**
+   * The entity type used for testing.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityType;
+
+  /**
+   * The entity type manager used for testing.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The ID of the type of the entity under test.
+   *
+   * @var string
+   */
+  protected $entityTypeId;
+
+  /**
+   * The UUID generator used for testing.
+   *
+   * @var \Drupal\Component\Uuid\UuidInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $uuid;
+
+  /**
+   * The typed configuration manager used for testing.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $typedConfigManager;
+
+  /**
+   * The typed configuration manager used for testing.
+   *
+   * @var \Drupal\Core\Config\Entity\ConfigEntityStorage|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $configEntityStorageInterface;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->entityTypeId = $this->randomMachineName();
+    $this->entityType = $this->createMock('\Drupal\Core\Entity\EntityTypeInterface');
+
+    $this->entityTypeManager = $this->createMock(EntityTypeManagerInterface::class);
+
+    $this->uuid = $this->createMock('\Drupal\Component\Uuid\UuidInterface');
+
+    $this->typedConfigManager = $this->createMock('Drupal\Core\Config\TypedConfigManagerInterface');
+
+    $this->configEntityStorageInterface = $this->createMock('Drupal\Core\Entity\EntityStorageInterface');
+
+    $container = new ContainerBuilder();
+    $container->set('entity_type.manager', $this->entityTypeManager);
+    $container->set('uuid', $this->uuid);
+    $container->set('config.typed', $this->typedConfigManager);
+    $container->set('config.storage', $this->configEntityStorageInterface);
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * @covers ::calculateDependencies
+   */
+  public function testCalculateDependencies(): void {
+    // Mock the interfaces necessary to create a dependency on a bundle entity.
+    $target_entity_type = $this->createMock('\Drupal\Core\Entity\EntityTypeInterface');
+    $target_entity_type->expects($this->any())
+      ->method('getBundleConfigDependency')
+      ->willReturn(['type' => 'config', 'name' => 'test.test_entity_type.id']);
+
+    $this->entityTypeManager->expects($this->any())
+      ->method('getDefinition')
+      ->with('test_entity_type')
+      ->willReturn($target_entity_type);
+
+    $config = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $dependencies = $config->calculateDependencies()->getDependencies();
+    $this->assertContains('test.test_entity_type.id', $dependencies['config']);
+  }
+
+  /**
+   * @covers ::id
+   */
+  public function testId(): void {
+    $config = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $this->assertSame('test_entity_type.test_bundle', $config->id());
+  }
+
+  /**
+   * @covers ::getTargetEntityTypeId
+   */
+  public function testTargetEntityTypeId(): void {
+    $config = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $this->assertSame('test_entity_type', $config->getTargetEntityTypeId());
+  }
+
+  /**
+   * @covers ::getTargetBundle
+   */
+  public function testTargetBundle(): void {
+    $config = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $this->assertSame('test_bundle', $config->getTargetBundle());
+  }
+
+  /**
+   * @covers ::getDefaultLangcode
+   * @covers ::setDefaultLangcode
+   *
+   * @dataProvider providerDefaultLangcode
+   */
+  public function testDefaultLangcode(ContentLanguageSettings $config, $expected): void {
+    $this->assertSame($expected, $config->getDefaultLangcode());
+  }
+
+  public static function providerDefaultLangcode() {
+    $langcode = Random::machineName();
+    $config = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $config->setDefaultLangcode($langcode);
+
+    $defaultConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_default_language_bundle',
+    ], 'language_content_settings');
+
+    return [
+      [$config, $langcode],
+      [$defaultConfig, LanguageInterface::LANGCODE_SITE_DEFAULT],
+    ];
+  }
+
+  /**
+   * @covers ::setLanguageAlterable
+   * @covers ::isLanguageAlterable
+   *
+   * @dataProvider providerLanguageAlterable
+   */
+  public function testLanguageAlterable(ContentLanguageSettings $config, $expected): void {
+    $this->assertSame($expected, $config->isLanguageAlterable());
+  }
+
+  public static function providerLanguageAlterable() {
+    $alterableConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $alterableConfig->setLanguageAlterable(TRUE);
+
+    $nonAlterableConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_fixed_language_bundle',
+    ], 'language_content_settings');
+    $nonAlterableConfig->setLanguageAlterable(FALSE);
+
+    $defaultConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_default_language_bundle',
+    ], 'language_content_settings');
+
+    return [
+      [$alterableConfig, TRUE],
+      [$nonAlterableConfig, FALSE],
+      [$defaultConfig, FALSE],
+    ];
+  }
+
+  /**
+   * @covers ::isDefaultConfiguration
+   *
+   * @dataProvider providerIsDefaultConfiguration
+   */
+  public function testIsDefaultConfiguration(ContentLanguageSettings $config, $expected): void {
+    $this->assertSame($expected, $config->isDefaultConfiguration());
+  }
+
+  public static function providerIsDefaultConfiguration() {
+    $alteredLanguage = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $alteredLanguage->setLanguageAlterable(TRUE);
+
+    $alteredDefaultLangcode = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_fixed_language_bundle',
+    ], 'language_content_settings');
+    $alteredDefaultLangcode->setDefaultLangcode(Random::machineName());
+
+    $defaultConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_default_language_bundle',
+    ], 'language_content_settings');
+
+    return [
+      [$alteredLanguage, FALSE],
+      [$alteredDefaultLangcode, FALSE],
+      [$defaultConfig, TRUE],
+    ];
+  }
+
+  /**
+   * @covers ::loadByEntityTypeBundle
+   *
+   * @dataProvider providerLoadByEntityTypeBundle
+   */
+  public function testLoadByEntityTypeBundle($config_id, ?ContentLanguageSettings $existing_config, $expected_langcode, $expected_language_alterable): void {
+    [$type, $bundle] = explode('.', $config_id);
+
+    $nullConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => $type,
+      'target_bundle' => $bundle,
+    ], 'language_content_settings');
+    $this->configEntityStorageInterface
+      ->expects($this->any())
+      ->method('load')
+      ->with($config_id)
+      ->willReturn($existing_config);
+    $this->configEntityStorageInterface
+      ->expects($this->any())
+      ->method('create')
+      ->willReturn($nullConfig);
+
+    $this->entityTypeManager
+      ->expects($this->any())
+      ->method('getStorage')
+      ->with('language_content_settings')
+      ->willReturn($this->configEntityStorageInterface);
+
+    $entity_type_repository = $this->createMock(EntityTypeRepositoryInterface::class);
+    $entity_type_repository->expects($this->any())
+      ->method('getEntityTypeFromClass')
+      ->with(ContentLanguageSettings::class)
+      ->willReturn('language_content_settings');
+
+    \Drupal::getContainer()->set('entity_type.repository', $entity_type_repository);
+
+    $config = ContentLanguageSettings::loadByEntityTypeBundle($type, $bundle);
+
+    $this->assertSame($expected_langcode, $config->getDefaultLangcode());
+    $this->assertSame($expected_language_alterable, $config->isLanguageAlterable());
+  }
+
+  public static function providerLoadByEntityTypeBundle() {
+    $alteredLanguage = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_bundle',
+    ], 'language_content_settings');
+    $alteredLanguage->setLanguageAlterable(TRUE);
+
+    $langcode = Random::machineName();
+    $alteredDefaultLangcode = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_fixed_language_bundle',
+    ], 'language_content_settings');
+    $alteredDefaultLangcode->setDefaultLangcode($langcode);
+
+    $defaultConfig = new ContentLanguageSettings([
+      'target_entity_type_id' => 'test_entity_type',
+      'target_bundle' => 'test_default_language_bundle',
+    ], 'language_content_settings');
+
+    return [
+      ['test_entity_type.test_bundle', $alteredLanguage, LanguageInterface::LANGCODE_SITE_DEFAULT, TRUE],
+      ['test_entity_type.test_fixed_language_bundle', $alteredDefaultLangcode, $langcode, FALSE],
+      ['test_entity_type.test_default_language_bundle', $defaultConfig, LanguageInterface::LANGCODE_SITE_DEFAULT, FALSE],
+      ['test_entity_type.null_bundle', NULL, LanguageInterface::LANGCODE_SITE_DEFAULT, FALSE],
+    ];
+  }
+
+}