first commit

core/modules/locale/tests/locale_test.js

@@ -0,0 +1,54 @@
+/**
+ * @file
+ * JavaScript for locale_test.module.
+ *
+ * @ignore
+ */
+
+Drupal.t("Standard Call t");
+Drupal
+  .
+  t
+  (
+    "Whitespace Call t"
+  )
+;
+
+Drupal.t('Single Quote t');
+Drupal.t('Single Quote \'Escaped\' t');
+Drupal.t('Single Quote ' + 'Concat ' + 'strings ' + 't');
+
+Drupal.t("Double Quote t");
+Drupal.t("Double Quote \"Escaped\" t");
+Drupal.t("Double Quote " + "Concat " + "strings " + "t");
+
+Drupal.t("Context Unquoted t", {}, {context: "Context string unquoted"});
+Drupal.t("Context Single Quoted t", {}, {'context': "Context string single quoted"});
+Drupal.t("Context Double Quoted t", {}, {"context": "Context string double quoted"});
+
+Drupal.t("Context !key Args t", {'!key': 'value'}, {context: "Context string"});
+
+Drupal.formatPlural(1, "Standard Call plural", "Standard Call @count plural");
+Drupal
+  .
+  formatPlural
+  (
+    1,
+    "Whitespace Call plural",
+    "Whitespace Call @count plural"
+  )
+;
+
+Drupal.formatPlural(1, 'Single Quote plural', 'Single Quote @count plural');
+Drupal.formatPlural(1, 'Single Quote \'Escaped\' plural', 'Single Quote \'Escaped\' @count plural');
+
+Drupal.formatPlural(1, "Double Quote plural", "Double Quote @count plural");
+Drupal.formatPlural(1, "Double Quote \"Escaped\" plural", "Double Quote \"Escaped\" @count plural");
+
+Drupal.formatPlural(1, "Context Unquoted plural", "Context Unquoted @count plural", {}, {context: "Context string unquoted"});
+Drupal.formatPlural(1, "Context Single Quoted plural", "Context Single Quoted @count plural", {}, {'context': "Context string single quoted"});
+Drupal.formatPlural(1, "Context Double Quoted plural", "Context Double Quoted @count plural", {}, {"context": "Context string double quoted"});
+
+Drupal.formatPlural(1, "Context !key Args plural", "Context !key Args @count plural", {'!key': 'value'}, {context: "Context string"});
+
+Drupal.formatPlural(1, "No count argument plural - singular", "No count argument plural - plural");

core/modules/locale/tests/modules/early_translation_test/early_translation_test.info.yml

@@ -0,0 +1,10 @@
+name: 'Early translation test'
+type: module
+description: 'Support module for testing early bootstrap getting of annotations with translations.'
+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/locale/tests/modules/early_translation_test/early_translation_test.services.yml

@@ -0,0 +1,6 @@
+services:
+  early_translation_test.authentication.early_translation_test:
+    class: Drupal\early_translation_test\Auth
+    arguments: ['@entity_type.manager']
+    tags:
+      - { name: authentication_provider, provider_id: 'early_translation_test', priority: 100 }

core/modules/locale/tests/modules/early_translation_test/src/Auth.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace Drupal\early_translation_test;
+
+use Drupal\Core\Authentication\AuthenticationProviderInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Test authentication provider.
+ */
+class Auth implements AuthenticationProviderInterface {
+
+  /**
+   * The user storage.
+   *
+   * @var \Drupal\user\UserStorageInterface
+   */
+  protected $userStorage;
+
+  /**
+   * Constructs an authentication provider object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    // Authentication providers are called early during in the bootstrap.
+    // Getting the user storage used to result in a circular reference since
+    // translation involves a call to \Drupal\locale\LocaleLookup that tries to
+    // get the user roles.
+    // @see https://www.drupal.org/node/2241461
+    $this->userStorage = $entity_type_manager->getStorage('user');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function authenticate(Request $request) {
+    return NULL;
+  }
+
+}

core/modules/locale/tests/modules/locale_test/locale_test.info.yml

@@ -0,0 +1,13 @@
+name: 'Locale test'
+type: module
+description: 'Support module for locale module testing.'
+package: Testing
+# version: '1.2'
+hidden: true
+'interface translation project': locale_test
+'interface translation server pattern': core/modules/locale/test/test.%language.po
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/locale/tests/modules/locale_test/locale_test.install

@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @file
+ * Install, update and uninstall functions for the locale_test module.
+ */
+
+/**
+ * Implements hook_uninstall().
+ */
+function locale_test_uninstall() {
+  // Clear variables.
+  \Drupal::state()->delete('locale.test_system_info_alter');
+  \Drupal::state()->delete('locale.test_projects_alter');
+}

core/modules/locale/tests/modules/locale_test/locale_test.module

@@ -0,0 +1,223 @@
+<?php
+
+/**
+ * @file
+ * Simulate a custom module with a local po file.
+ */
+
+use Drupal\Core\Url;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\StreamWrapper\PublicStream;
+
+/**
+ * Implements hook_system_info_alter().
+ *
+ * Make the test scripts to be believe this is not a hidden test module, but
+ * a regular custom module.
+ */
+function locale_test_system_info_alter(&$info, Extension $file, $type) {
+  // Only modify the system info if required.
+  // By default the locale_test modules are hidden and have a project specified.
+  // To test the module detection process by locale_project_list() the
+  // test modules should mimic a custom module. I.e. be non-hidden.
+  if (\Drupal::state()->get('locale.test_system_info_alter')) {
+    if ($file->getName() == 'locale_test' || $file->getName() == 'locale_test_translate') {
+      // Don't hide the module.
+      $info['hidden'] = FALSE;
+    }
+  }
+
+  // Alter the name and the core version of the project. This should not affect
+  // the locale project information.
+  if (\Drupal::state()->get('locale.test_system_info_alter_name_core')) {
+    if ($file->getName() == 'locale_test') {
+      $info['core'] = '8.6.7';
+      $info['name'] = 'locale_test_alter';
+    }
+  }
+}
+
+/**
+ * Implements hook_locale_translation_projects_alter().
+ *
+ * The translation status process by default checks the status of the installed
+ * projects. This function replaces the data of the installed modules by a
+ * predefined set of modules with fixed file names and release versions. Project
+ * names, versions, timestamps etc must be fixed because they must match the
+ * files created by the test script.
+ *
+ * The "locale.test_projects_alter" state variable must be set by the
+ * test script in order for this hook to take effect.
+ */
+function locale_test_locale_translation_projects_alter(&$projects) {
+  // Drupal core should not be translated. By overriding the server pattern we
+  // make sure that no translation for drupal core will be found and that the
+  // translation update system will not go out to l.d.o to check.
+  $projects['drupal']['server_pattern'] = 'translations://';
+
+  if (\Drupal::state()->get('locale.remove_core_project')) {
+    unset($projects['drupal']);
+  }
+
+  if (\Drupal::state()->get('locale.test_projects_alter')) {
+
+    // Instead of the default ftp.drupal.org we use the file system of the test
+    // instance to simulate a remote file location.
+    $url = Url::fromRoute('<front>', [], ['absolute' => TRUE])->toString();
+    $remote_url = $url . PublicStream::basePath() . '/remote/';
+
+    // Completely replace the project data with a set of test projects.
+    $projects = [
+      'contrib_module_one' => [
+        'name' => 'contrib_module_one',
+        'info' => [
+          'name' => 'Contributed module one',
+          'interface translation server pattern' => $remote_url . '%core/%project/%project-%version.%language._po',
+          'package' => 'Other',
+          'version' => '8.x-1.1',
+          'project' => 'contrib_module_one',
+          'datestamp' => '1344471537',
+          '_info_file_ctime' => 1348767306,
+        ],
+        'datestamp' => '1344471537',
+        'project_type' => 'module',
+        'project_status' => TRUE,
+      ],
+      'contrib_module_two' => [
+        'name' => 'contrib_module_two',
+        'info' => [
+          'name' => 'Contributed module two',
+          'interface translation server pattern' => $remote_url . '%core/%project/%project-%version.%language._po',
+          'package' => 'Other',
+          'version' => '8.x-2.0-beta4',
+          'project' => 'contrib_module_two',
+          'datestamp' => '1344471537',
+          '_info_file_ctime' => 1348767306,
+        ],
+        'datestamp' => '1344471537',
+        'project_type' => 'module',
+        'project_status' => TRUE,
+      ],
+      'contrib_module_three' => [
+        'name' => 'contrib_module_three',
+        'info' => [
+          'name' => 'Contributed module three',
+          'interface translation server pattern' => $remote_url . '%core/%project/%project-%version.%language._po',
+          'package' => 'Other',
+          'version' => '8.x-1.0',
+          'project' => 'contrib_module_three',
+          'datestamp' => '1344471537',
+          '_info_file_ctime' => 1348767306,
+        ],
+        'datestamp' => '1344471537',
+        'project_type' => 'module',
+        'project_status' => TRUE,
+      ],
+      'locale_test' => [
+        'name' => 'locale_test',
+        'info' => [
+          'name' => 'Locale test',
+          'interface translation project' => 'locale_test',
+          'interface translation server pattern' => 'core/modules/locale/tests/test.%language.po',
+          'package' => 'Other',
+          'version' => NULL,
+          'project' => 'locale_test',
+          '_info_file_ctime' => 1348767306,
+          'datestamp' => 0,
+        ],
+        'datestamp' => 0,
+        'project_type' => 'module',
+        'project_status' => TRUE,
+      ],
+      'custom_module_one' => [
+        'name' => 'custom_module_one',
+        'info' => [
+          'name' => 'Custom module one',
+          'interface translation project' => 'custom_module_one',
+          'interface translation server pattern' => 'translations://custom_module_one.%language.po',
+          'package' => 'Other',
+          'version' => NULL,
+          'project' => 'custom_module_one',
+          '_info_file_ctime' => 1348767306,
+          'datestamp' => 0,
+        ],
+        'datestamp' => 0,
+        'project_type' => 'module',
+        'project_status' => TRUE,
+      ],
+    ];
+  }
+}
+
+/**
+ * Implements hook_language_fallback_candidates_OPERATION_alter().
+ */
+function locale_test_language_fallback_candidates_locale_lookup_alter(array &$candidates, array $context) {
+  \Drupal::state()->set('locale.test_language_fallback_candidates_locale_lookup_alter_candidates', $candidates);
+  \Drupal::state()->set('locale.test_language_fallback_candidates_locale_lookup_alter_context', $context);
+}
+
+/**
+ * Implements hook_theme().
+ */
+function locale_test_theme() {
+  $return = [];
+
+  $return['locale_test_tokenized'] = [
+    'variable' => ['content' => ''],
+  ];
+
+  return $return;
+}
+
+/**
+ * Implements hook_token_info().
+ */
+function locale_test_token_info() {
+  $info = [];
+
+  $info['types']['locale_test'] = [
+    'name' => t('Locale test'),
+    'description' => t('Locale test'),
+  ];
+
+  $info['tokens']['locale_test']['security_test1'] = [
+    'type' => 'text',
+    'name' => t('Security test 1'),
+  ];
+  $info['tokens']['locale_test']['security_test2'] = [
+    'type' => 'text',
+    'name' => t('Security test 2'),
+  ];
+
+  return $info;
+}
+
+/**
+ * Implements hook_tokens().
+ */
+function locale_test_tokens($type, $tokens, array $data = [], array $options = []) {
+  $return = [];
+  if ($type == 'locale_test') {
+    foreach ($tokens as $name => $original) {
+      switch ($name) {
+        case 'security_test1':
+          $return[$original] = "javascript:alert('Hello!');";
+          break;
+
+        case 'security_test2':
+          $return[$original] = "<script>alert('Hello!');</script>";
+          break;
+      }
+    }
+  }
+
+  return $return;
+}
+
+/**
+ * Implements hook_countries_alter().
+ */
+function locale_test_countries_alter(&$countries) {
+  $countries['EB'] = 'Elbonia';
+}

core/modules/locale/tests/modules/locale_test/templates/locale-test-tokenized.html.twig

@@ -0,0 +1 @@
+{{ content }}

core/modules/locale/tests/modules/locale_test_development_release/locale_test_development_release.info.yml

@@ -0,0 +1,11 @@
+name: 'Locale Test Development Release'
+type: module
+description: 'Helper module to test the behavior when the core version is a development release.'
+package: Testing
+# version: VERSION
+hidden: true
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/locale/tests/modules/locale_test_development_release/locale_test_development_release.module

@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @file
+ * Simulate a Drupal version.
+ */
+
+use Drupal\Core\Extension\Extension;
+
+/**
+ * Implements hook_system_info_alter().
+ *
+ * Change the core version number to a development one for testing.
+ * 8.0.0-alpha102-dev is the simulated version.
+ */
+function locale_test_development_release_system_info_alter(&$info, Extension $file, $type) {
+  if (isset($info['package']) && $info['package'] == 'Core') {
+    $info['version'] = '8.0.0-alpha102-dev';
+  }
+}
+
+/**
+ * Implements hook_locale_translation_projects_alter().
+ *
+ * Add a contrib module with a dev release to list of translatable modules.
+ */
+function locale_test_development_release_locale_translation_projects_alter(&$projects) {
+  $projects['contrib'] = [
+    'name' => 'contrib',
+    'info' => [
+      'name' => 'Contributed module',
+      'package' => 'Other',
+      'version' => '12.x-10.4-unstable11+14-dev',
+      'project' => 'contrib_module',
+      'datestamp' => '0',
+      '_info_file_ctime' => 1442933959,
+    ],
+    'datestamp' => '0',
+    'project_type' => 'module',
+    'project_status' => TRUE,
+  ];
+}

core/modules/locale/tests/modules/locale_test_translate/locale_test_translate.info.yml

@@ -0,0 +1,13 @@
+name: 'Locale test translate'
+type: module
+description: 'Translation test module for locale module testing.'
+package: Testing
+# version: '1.3'
+hidden: true
+'interface translation project': locale_test_translate
+'interface translation server pattern': core/modules/locale/tests/test.%language.po
+
+# Information added by Drupal.org packaging script on 2024-07-04
+version: '10.3.1'
+project: 'drupal'
+datestamp: 1720094222

core/modules/locale/tests/modules/locale_test_translate/locale_test_translate.module

@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @file
+ * Simulates a custom module with a local po file.
+ */
+
+use Drupal\Core\Extension\Extension;
+
+/**
+ * Implements hook_system_info_alter().
+ *
+ * By default this modules is hidden but once enabled it behaves like a normal
+ * (not hidden) module. This hook implementation changes the .info.yml data by
+ * setting the hidden status to FALSE.
+ */
+function locale_test_translate_system_info_alter(&$info, Extension $file, $type) {
+  if ($file->getName() == 'locale_test_translate') {
+    // Don't hide the module.
+    $info['hidden'] = FALSE;
+  }
+}
+
+/**
+ * Implements hook_modules_installed().
+ *
+ * @see \Drupal\Tests\locale\Functional\LocaleConfigTranslationImportTest::testConfigTranslationWithForeignLanguageDefault
+ */
+function locale_test_translate_modules_installed($modules, $is_syncing) {
+  // Ensure that writing to configuration during install does not cause
+  // \Drupal\locale\LocaleConfigSubscriber to create incorrect translations due
+  // the configuration langcode and data being out-of-sync.
+  \Drupal::configFactory()->getEditable('locale_test_translate.settings')->set('key_set_during_install', TRUE)->save();
+}

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

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

core/modules/locale/tests/src/Functional/LocaleConfigTranslationImportTest.php

@@ -0,0 +1,371 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests translation update's effects on configuration translations.
+ *
+ * @group locale
+ */
+class LocaleConfigTranslationImportTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'locale_test_translate'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+  }
+
+  /**
+   * Tests update changes configuration translations if enabled after language.
+   */
+  public function testConfigTranslationImport(): void {
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'administer permissions',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Add a language. The Afrikaans translation file of locale_test_translate
+    // (test.af.po) has been prepared with a configuration translation.
+    ConfigurableLanguage::createFromLangcode('af')->save();
+
+    // Enable locale module.
+    $this->container->get('module_installer')->install(['locale']);
+    $this->resetAll();
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // Add translation permissions now that the locale module has been enabled.
+    $edit = [
+      'authenticated[translate interface]' => 'translate interface',
+    ];
+    $this->drupalGet('admin/people/permissions');
+    $this->submitForm($edit, 'Save permissions');
+
+    // Check and update the translation status. This will import the Afrikaans
+    // translations of locale_test_translate module.
+    $this->drupalGet('admin/reports/translations/check');
+
+    // Override the Drupal core translation status to be up to date.
+    // Drupal core should not be a subject in this test.
+    $status = locale_translation_get_status();
+    $status['drupal']['af']->type = 'current';
+    \Drupal::state()->set('locale.translation_status', $status);
+
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check if configuration translations have been imported.
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'system.maintenance');
+    // cSpell:disable-next-line
+    $this->assertEquals('Ons is tans besig met onderhoud op @site. Wees asseblief geduldig, ons sal binnekort weer terug wees.', $override->get('message'));
+
+    // Ensure that \Drupal\locale\LocaleConfigSubscriber::onConfigSave() works
+    // as expected during a configuration install that installs locale.
+    /** @var \Drupal\Core\Config\FileStorage $sync */
+    $sync = $this->container->get('config.storage.sync');
+    $this->copyConfig($this->container->get('config.storage'), $sync);
+
+    // Add our own translation to the config that will be imported.
+    $af_sync = $sync->createCollection('language.af');
+    $data = $af_sync->read('system.maintenance');
+    $data['message'] = 'Test af message';
+    $af_sync->write('system.maintenance', $data);
+
+    // Uninstall locale module.
+    $this->container->get('module_installer')->uninstall(['locale_test_translate']);
+    $this->container->get('module_installer')->uninstall(['locale']);
+    $this->resetAll();
+
+    $this->configImporter()->import();
+
+    $this->drupalGet('admin/reports/translations/check');
+    $status = locale_translation_get_status();
+    $status['drupal']['af']->type = 'current';
+    \Drupal::state()->set('locale.translation_status', $status);
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check if configuration translations have been imported.
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'system.maintenance');
+    $this->assertEquals('Test af message', $override->get('message'));
+  }
+
+  /**
+   * Tests update changes configuration translations if enabled after language.
+   */
+  public function testConfigTranslationModuleInstall(): void {
+
+    // Enable locale, block and config_translation modules.
+    $this->container->get('module_installer')->install(['block', 'config_translation']);
+    $this->resetAll();
+
+    // The testing profile overrides locale.settings to disable translation
+    // import. Test that this override is in place.
+    $this->assertFalse($this->config('locale.settings')->get('translation.import_enabled'), 'Translations imports are disabled by default in the Testing profile.');
+
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'administer permissions',
+      'translate configuration',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // Add predefined language.
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'af'], 'Add language');
+
+    // Add the system branding block to the page.
+    $this->drupalPlaceBlock('system_branding_block', ['region' => 'header', 'id' => 'site_branding']);
+    $this->drupalGet('admin/config/system/site-information');
+    $this->submitForm(['site_slogan' => 'Test site slogan'], 'Save configuration');
+    $this->drupalGet('admin/config/system/site-information/translate/af/edit');
+    $this->submitForm([
+      'translation[config_names][system.site][slogan]' => 'Test site slogan in Afrikaans',
+    ], 'Save translation');
+
+    // Get the front page and ensure that the translated configuration appears.
+    $this->drupalGet('af');
+    $this->assertSession()->pageTextContains('Test site slogan in Afrikaans');
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    $this->assertEquals('Locale can translate Afrikaans', $override->get('translatable_default_with_translation'));
+
+    // Update test configuration.
+    $override
+      ->set('translatable_no_default', 'This translation is preserved')
+      ->set('translatable_default_with_translation', 'This translation is preserved')
+      ->set('translatable_default_with_no_translation', 'This translation is preserved')
+      ->save();
+
+    // Install any module.
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[dblog][enable]' => 'dblog'], 'Install');
+    $this->assertSession()->pageTextContains('Module Database Logging has been installed.');
+
+    // Get the front page and ensure that the translated configuration still
+    // appears.
+    $this->drupalGet('af');
+    $this->assertSession()->pageTextContains('Test site slogan in Afrikaans');
+
+    $this->rebuildContainer();
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    $expected = [
+      'translatable_no_default' => 'This translation is preserved',
+      'translatable_default_with_translation' => 'This translation is preserved',
+      'translatable_default_with_no_translation' => 'This translation is preserved',
+    ];
+    $this->assertEquals($expected, $override->get());
+  }
+
+  /**
+   * Tests removing a string from Locale deletes configuration translations.
+   */
+  public function testLocaleRemovalAndConfigOverrideDelete(): void {
+    // Enable the locale module.
+    $this->container->get('module_installer')->install(['locale']);
+    $this->resetAll();
+
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'administer permissions',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // Add predefined language.
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'af'], 'Add language');
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    $this->assertEquals(['translatable_default_with_translation' => 'Locale can translate Afrikaans'], $override->get());
+
+    // Remove the string from translation to simulate a Locale removal. Note
+    // that is no current way of doing this in the UI.
+    $locale_storage = \Drupal::service('locale.storage');
+    $string = $locale_storage->findString(['source' => 'Locale can translate']);
+    \Drupal::service('locale.storage')->delete($string);
+    // Force a rebuild of config translations.
+    $count = \Drupal::service('locale.config_manager')->updateConfigTranslations(['locale_test_translate.settings'], ['af']);
+    $this->assertEquals(1, $count, 'Correct count of updated translations');
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    $this->assertEquals([], $override->get());
+    $this->assertTrue($override->isNew(), 'The configuration override was deleted when the Locale string was deleted.');
+  }
+
+  /**
+   * Tests removing a string from Locale changes configuration translations.
+   */
+  public function testLocaleRemovalAndConfigOverridePreserve(): void {
+    // Enable the locale module.
+    $this->container->get('module_installer')->install(['locale']);
+    $this->resetAll();
+
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'administer permissions',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // Add predefined language.
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'af'], 'Add language');
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    // Update test configuration.
+    $override
+      ->set('translatable_no_default', 'This translation is preserved')
+      ->set('translatable_default_with_no_translation', 'This translation is preserved')
+      ->save();
+    $expected = [
+      'translatable_default_with_translation' => 'Locale can translate Afrikaans',
+      'translatable_no_default' => 'This translation is preserved',
+      'translatable_default_with_no_translation' => 'This translation is preserved',
+    ];
+    $this->assertEquals($expected, $override->get());
+
+    // Set the translated string to empty.
+    $search = [
+      'string' => 'Locale can translate',
+      'langcode' => 'af',
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => '',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('af', 'locale_test_translate.settings');
+    $expected = [
+      'translatable_no_default' => 'This translation is preserved',
+      'translatable_default_with_no_translation' => 'This translation is preserved',
+    ];
+    $this->assertEquals($expected, $override->get());
+  }
+
+  /**
+   * Tests setting a non-English language as default and importing configuration.
+   */
+  public function testConfigTranslationWithNonEnglishLanguageDefault(): void {
+    /** @var \Drupal\Core\Extension\ModuleInstallerInterface $module_installer */
+    $module_installer = $this->container->get('module_installer');
+    ConfigurableLanguage::createFromLangcode('af')->save();
+
+    $module_installer->install(['locale']);
+    $this->resetAll();
+    /** @var \Drupal\locale\StringStorageInterface $local_storage */
+    $local_storage = $this->container->get('locale.storage');
+
+    $source_string = 'Locale can translate';
+    $translation_string = 'Locale can translate Afrikaans';
+
+    // Create a translation for the "Locale can translate" string, this string
+    // can be found in the "locale_test_translate" module's install config.
+    $source = $local_storage->createString([
+      'source' => $source_string,
+    ])->save();
+    $local_storage->createTranslation([
+      'lid' => $source->getId(),
+      'language' => 'af',
+      'translation' => $translation_string,
+    ])->save();
+
+    // Verify that we can find the newly added string translation, it is not a
+    // customized translation.
+    $translation = $local_storage->findTranslation([
+      'source' => $source_string,
+      'language' => 'af',
+    ]);
+    $this->assertEquals($translation_string, $translation->getString());
+    $this->assertFalse((bool) $translation->customized);
+
+    // Uninstall the "locale_test_translate" module, verify that we can still
+    // find the string translation.
+    $module_installer->uninstall(['locale_test_translate']);
+    $this->resetAll();
+    $translation = $local_storage->findTranslation([
+      'source' => $source_string,
+      'language' => 'af',
+    ]);
+    $this->assertEquals($translation_string, $translation->getString());
+
+    // Set the default language to "Afrikaans" and re-enable the
+    // "locale_test_translate" module.
+    $this->config('system.site')->set('default_langcode', 'af')->save();
+    $module_installer->install(['locale_test_translate']);
+    $this->resetAll();
+
+    // Verify that enabling the "locale_test_translate" module didn't cause
+    // the string translation to be overwritten.
+    $translation = $local_storage->findTranslation([
+      'source' => $source_string,
+      'language' => 'af',
+    ]);
+    $this->assertEquals($translation_string, $translation->getString());
+    $this->assertFalse((bool) $translation->customized);
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleConfigTranslationTest.php

@@ -0,0 +1,285 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Drupal\locale\StringStorageInterface;
+use Drupal\Core\Language\LanguageInterface;
+
+/**
+ * Tests translation of configuration strings.
+ *
+ * @group locale
+ */
+class LocaleConfigTranslationTest extends BrowserTestBase {
+
+  /**
+   * The language code used.
+   *
+   * @var string
+   */
+  protected $langcode;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'contact', 'contact_test'];
+
+  /**
+   * {@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');
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // Add custom language.
+    $this->langcode = 'xx';
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+      'administer modules',
+      'access site-wide contact form',
+      'administer contact forms',
+      'administer site configuration',
+    ]);
+    $this->drupalLogin($admin_user);
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $this->langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    // Set path prefix.
+    $edit = ["prefix[$this->langcode]" => $this->langcode];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+  }
+
+  /**
+   * Tests basic configuration translation.
+   */
+  public function testConfigTranslation(): void {
+    // Check that the maintenance message exists and create translation for it.
+    $source = '@site is currently under maintenance. We should be back shortly. Thank you for your patience.';
+    $string = $this->storage->findString(['source' => $source, 'context' => '', 'type' => 'configuration']);
+    $this->assertNotEmpty($string, 'Configuration strings have been created upon installation.');
+
+    // Translate using the UI so configuration is refreshed.
+    $message = $this->randomMachineName(20);
+    $search = [
+      'string' => $string->source,
+      'langcode' => $this->langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $message,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Get translation and check we've only got the message.
+    $translation = \Drupal::languageManager()->getLanguageConfigOverride($this->langcode, 'system.maintenance')->get();
+    $this->assertCount(1, $translation, 'Got the right number of properties after translation.');
+    $this->assertEquals($message, $translation['message']);
+
+    // Check default medium date format exists and create a translation for it.
+    $string = $this->storage->findString(['source' => 'D, m/d/Y - H:i', 'context' => 'PHP date format', 'type' => 'configuration']);
+    $this->assertNotEmpty($string, 'Configuration date formats have been created upon installation.');
+
+    // Translate using the UI so configuration is refreshed.
+    $search = [
+      'string' => $string->source,
+      'langcode' => $this->langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => 'D',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    $translation = \Drupal::languageManager()->getLanguageConfigOverride($this->langcode, 'core.date_format.medium')->get();
+    $this->assertEquals('D', $translation['pattern'], 'Got the right date format pattern after translation.');
+
+    // Formatting the date 8 / 27 / 1985 @ 13:37 EST with pattern D should
+    // display "Tue".
+    $formatted_date = $this->container->get('date.formatter')->format(494015820, $type = 'medium', NULL, 'America/New_York', $this->langcode);
+    $this->assertEquals('Tue', $formatted_date, 'Got the right formatted date using the date format translation pattern.');
+
+    // Assert strings from image module config are not available.
+    $string = $this->storage->findString(['source' => 'Medium (220×220)', 'context' => '', 'type' => 'configuration']);
+    $this->assertNull($string, 'Configuration strings have been created upon installation.');
+
+    // Enable the image module.
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[image][enable]' => "1"], 'Install');
+    $this->rebuildContainer();
+
+    $string = $this->storage->findString(['source' => 'Medium (220×220)', 'context' => '', 'type' => 'configuration']);
+    $this->assertNotEmpty($string, 'Configuration strings have been created upon installation.');
+    $locations = $string->getLocations();
+    // Check the configuration string has been created with the right location.
+    $this->assertArrayHasKey('configuration', $locations);
+    $this->assertArrayHasKey('image.style.medium', $locations['configuration']);
+
+    // Check the string is unique and has no translation yet.
+    $translations = $this->storage->getTranslations(['language' => $this->langcode, 'type' => 'configuration', 'name' => 'image.style.medium']);
+    $this->assertCount(1, $translations);
+    $translation = reset($translations);
+    $this->assertEquals($string->source, $translation->source);
+    $this->assertEmpty($translation->translation);
+
+    // Translate using the UI so configuration is refreshed.
+    $image_style_label = $this->randomMachineName(20);
+    $search = [
+      'string' => $string->source,
+      'langcode' => $this->langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $image_style_label,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Check the right single translation has been created.
+    $translations = $this->storage->getTranslations(['language' => $this->langcode, 'type' => 'configuration', 'name' => 'image.style.medium']);
+    $translation = reset($translations);
+    $this->assertCount(1, $translations, 'Got only one translation for image configuration.');
+    $this->assertEquals($string->source, $translation->source);
+    $this->assertEquals($image_style_label, $translation->translation);
+
+    // Try more complex configuration data.
+    $translation = \Drupal::languageManager()->getLanguageConfigOverride($this->langcode, 'image.style.medium')->get();
+    $this->assertEquals($image_style_label, $translation['label'], 'Got the right translation for image style name after translation');
+
+    // Uninstall the module.
+    $this->drupalGet('admin/modules/uninstall');
+    $this->submitForm(['uninstall[image]' => "image"], 'Uninstall');
+    $this->submitForm([], 'Uninstall');
+
+    // Ensure that the translated configuration has been removed.
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('xx', 'image.style.medium');
+    $this->assertTrue($override->isNew(), 'Translated configuration for image module removed.');
+
+    // Translate default category using the UI so configuration is refreshed.
+    $category_label = $this->randomMachineName(20);
+    $search = [
+      'string' => 'Website feedback',
+      'langcode' => $this->langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $category_label,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Check if this category displayed in this language will use the
+    // translation. This test ensures the entity loaded from the request
+    // upcasting will already work.
+    $this->drupalGet($this->langcode . '/contact/feedback');
+    $this->assertSession()->pageTextContains($category_label);
+
+    // Check if the UI does not show the translated String.
+    $this->drupalGet('admin/structure/contact/manage/feedback');
+    $this->assertSession()->fieldValueEquals('edit-label', 'Website feedback');
+  }
+
+  /**
+   * Tests translatability of optional configuration in locale.
+   */
+  public function testOptionalConfiguration(): void {
+    $this->assertNodeConfig(FALSE, FALSE);
+    // Enable the node module.
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[node][enable]' => "1"], 'Install');
+    $this->submitForm([], 'Continue');
+    $this->rebuildContainer();
+    $this->assertNodeConfig(TRUE, FALSE);
+    // Enable the views module (which node provides some optional config for).
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[views][enable]' => "1"], 'Install');
+    $this->rebuildContainer();
+    $this->assertNodeConfig(TRUE, TRUE);
+  }
+
+  /**
+   * Check that node configuration source strings are made available in locale.
+   *
+   * @param bool $required
+   *   Whether to assume a sample of the required default configuration is
+   *   present.
+   * @param bool $optional
+   *   Whether to assume a sample of the optional default configuration is
+   *   present.
+   *
+   * @internal
+   */
+  protected function assertNodeConfig(bool $required, bool $optional): void {
+    // Check the required default configuration in node module.
+    $string = $this->storage->findString(['source' => 'Make content sticky', 'context' => '', 'type' => 'configuration']);
+    if ($required) {
+      $this->assertFalse($this->config('system.action.node_make_sticky_action')->isNew());
+      $this->assertNotEmpty($string, 'Node action text can be found with node module.');
+    }
+    else {
+      $this->assertTrue($this->config('system.action.node_make_sticky_action')->isNew());
+      $this->assertNull($string, 'Node action text can not be found without node module.');
+    }
+
+    // Check the optional default configuration in node module.
+    $string = $this->storage->findString(['source' => 'No front page content has been created yet.<br/>Follow the <a target="_blank" href="https://www.drupal.org/docs/user_guide/en/index.html">User Guide</a> to start building your site.', 'context' => '', 'type' => 'configuration']);
+    if ($optional) {
+      $this->assertFalse($this->config('views.view.frontpage')->isNew());
+      $this->assertNotEmpty($string, 'Node view text can be found with node and views modules.');
+    }
+    else {
+      $this->assertTrue($this->config('views.view.frontpage')->isNew());
+      $this->assertNull($string, 'Node view text can not be found without node and/or views modules.');
+    }
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleContentTest.php

@@ -0,0 +1,240 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\node\NodeInterface;
+
+/**
+ * Tests multilingual support for content types and individual nodes.
+ *
+ * @group locale
+ */
+class LocaleContentTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['node', 'locale'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Verifies that machine name fields are always LTR.
+   */
+  public function testMachineNameLTR(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'administer content types',
+      'access administration pages',
+      'administer site configuration',
+    ]);
+
+    // Log in as admin.
+    $this->drupalLogin($admin_user);
+
+    // Verify that the machine name field is LTR for a new content type.
+    $this->drupalGet('admin/structure/types/add');
+    $type = $this->assertSession()->fieldExists('type');
+    $this->assertSame('ltr', $type->getAttribute('dir'));
+
+    // Install the Arabic language (which is RTL) and configure as the default.
+    $edit = [];
+    $edit['predefined_langcode'] = 'ar';
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    $edit = [
+      'site_default_language' => 'ar',
+    ];
+    $this->drupalGet('admin/config/regional/language');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Verify that the machine name field is still LTR for a new content type.
+    $this->drupalGet('admin/structure/types/add');
+    $type = $this->assertSession()->fieldExists('type');
+    $this->assertSame('ltr', $type->getAttribute('dir'));
+  }
+
+  /**
+   * Tests if a content type can be set to multilingual and language is present.
+   */
+  public function testContentTypeLanguageConfiguration(): void {
+    $type1 = $this->drupalCreateContentType();
+    $type2 = $this->drupalCreateContentType();
+
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'administer content types',
+      'access administration pages',
+    ]);
+    // User to create a node.
+    $web_user = $this->drupalCreateUser([
+      "create {$type1->id()} content",
+      "create {$type2->id()} content",
+      "edit any {$type2->id()} content",
+    ]);
+
+    // Add custom language.
+    $this->drupalLogin($admin_user);
+    // Code for the language.
+    $langcode = 'xx';
+    // The English name for the language.
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Set the content type to use multilingual support.
+    $this->drupalGet("admin/structure/types/manage/{$type2->id()}");
+    $this->assertSession()->pageTextContains('Language settings');
+    $edit = [
+      'language_configuration[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet("admin/structure/types/manage/{$type2->id()}");
+    $this->submitForm($edit, 'Save');
+    $this->assertSession()->pageTextContains("The content type {$type2->label()} has been updated.");
+    $this->drupalLogout();
+    \Drupal::languageManager()->reset();
+
+    // Verify language selection is not present on the node add form.
+    $this->drupalLogin($web_user);
+    $this->drupalGet("node/add/{$type1->id()}");
+    // Verify language select list is not present.
+    $this->assertSession()->fieldNotExists('langcode[0][value]');
+
+    // Verify language selection appears on the node add form.
+    $this->drupalGet("node/add/{$type2->id()}");
+    // Verify language select list is present.
+    $this->assertSession()->fieldExists('langcode[0][value]');
+    // Ensure language appears.
+    $this->assertSession()->pageTextContains($name);
+
+    // Create a node.
+    $node_title = $this->randomMachineName();
+    $node_body = $this->randomMachineName();
+    $edit = [
+      'type' => $type2->id(),
+      'title' => $node_title,
+      'body' => [['value' => $node_body]],
+      'langcode' => $langcode,
+    ];
+    $node = $this->drupalCreateNode($edit);
+    // Edit the content and ensure correct language is selected.
+    $path = 'node/' . $node->id() . '/edit';
+    $this->drupalGet($path);
+    $this->assertSession()->responseContains('<option value="' . $langcode . '" selected="selected">' . $name . '</option>');
+    // Ensure we can change the node language.
+    $edit = [
+      'langcode[0][value]' => 'en',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save');
+    $this->assertSession()->pageTextContains($node_title . ' has been updated.');
+
+    // Verify that the creation message contains a link to a node.
+    $xpath = $this->assertSession()->buildXPathQuery('//div[@data-drupal-messages]//a[contains(@href, :href)]', [
+      ':href' => 'node/' . $node->id(),
+    ]);
+    $this->assertSession()->elementExists('xpath', $xpath);
+
+    $this->drupalLogout();
+  }
+
+  /**
+   * Tests if a dir and lang tags exist in node's attributes.
+   */
+  public function testContentTypeDirLang(): void {
+    $type = $this->drupalCreateContentType();
+
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'administer content types',
+      'access administration pages',
+    ]);
+    // User to create a node.
+    $web_user = $this->drupalCreateUser([
+      "create {$type->id()} content",
+      "edit own {$type->id()} content",
+    ]);
+
+    // Log in as admin.
+    $this->drupalLogin($admin_user);
+
+    // Install Arabic language.
+    $edit = [];
+    $edit['predefined_langcode'] = 'ar';
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Install Spanish language.
+    $edit = [];
+    $edit['predefined_langcode'] = 'es';
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    \Drupal::languageManager()->reset();
+
+    // Set the content type to use multilingual support.
+    $this->drupalGet("admin/structure/types/manage/{$type->id()}");
+    $edit = [
+      'language_configuration[language_alterable]' => TRUE,
+    ];
+    $this->drupalGet("admin/structure/types/manage/{$type->id()}");
+    $this->submitForm($edit, 'Save');
+    $this->assertSession()->pageTextContains("The content type {$type->label()} has been updated.");
+    $this->drupalLogout();
+
+    // Log in as web user to add new node.
+    $this->drupalLogin($web_user);
+
+    // Create three nodes: English, Arabic and Spanish.
+    $nodes = [];
+    foreach (['en', 'es', 'ar'] as $langcode) {
+      $nodes[$langcode] = $this->drupalCreateNode([
+        'langcode' => $langcode,
+        'type' => $type->id(),
+        'promote' => NodeInterface::PROMOTED,
+      ]);
+    }
+
+    // Check if English node does not have lang tag.
+    $this->drupalGet('node/' . $nodes['en']->id());
+    $element = $this->cssSelect('article[lang="en"]');
+    $this->assertEmpty($element, 'The lang tag has not been assigned to the English node.');
+
+    // Check if English node does not have dir tag.
+    $element = $this->cssSelect('article[dir="ltr"]');
+    $this->assertEmpty($element, 'The dir tag has not been assigned to the English node.');
+
+    // Check if Arabic node has lang="ar" & dir="rtl" tags.
+    $this->drupalGet('node/' . $nodes['ar']->id());
+    $element = $this->cssSelect('article[lang="ar"][dir="rtl"]');
+    $this->assertNotEmpty($element, 'The lang and dir tags have been assigned correctly to the Arabic node.');
+
+    // Check if Spanish node has lang="es" tag.
+    $this->drupalGet('node/' . $nodes['es']->id());
+    $element = $this->cssSelect('article[lang="es"]');
+    $this->assertNotEmpty($element, 'The lang tag has been assigned correctly to the Spanish node.');
+
+    // Check if Spanish node does not have dir="ltr" tag.
+    $element = $this->cssSelect('article[lang="es"][dir="ltr"]');
+    $this->assertEmpty($element, 'The dir tag has not been assigned to the Spanish node.');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleExportTest.php

@@ -0,0 +1,194 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\File\FileExists;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore janvier lundi
+
+/**
+ * Tests the exportation of locale files.
+ *
+ * @group locale
+ */
+class LocaleExportTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * A user able to create languages and export translations.
+   */
+  protected $adminUser = NULL;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->adminUser = $this->drupalCreateUser([
+      'administer languages',
+      'translate interface',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($this->adminUser);
+
+    // Copy test po files to the translations directory.
+    \Drupal::service('file_system')->copy(__DIR__ . '/../../../tests/test.de.po', 'translations://', FileExists::Replace);
+    \Drupal::service('file_system')->copy(__DIR__ . '/../../../tests/test.xx.po', 'translations://', FileExists::Replace);
+  }
+
+  /**
+   * Tests exportation of translations.
+   */
+  public function testExportTranslation(): void {
+    $file_system = \Drupal::service('file_system');
+    // First import some known translations.
+    // This will also automatically add the 'fr' language.
+    $name = $file_system->tempnam('temporary://', "po_") . '.po';
+    file_put_contents($name, $this->getPoFile());
+    $this->drupalGet('admin/config/regional/translate/import');
+    $this->submitForm([
+      'langcode' => 'fr',
+      'files[file]' => $name,
+    ], 'Import');
+    $file_system->unlink($name);
+
+    // Get the French translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm(['langcode' => 'fr'], 'Export');
+
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# French translation of Drupal');
+    // Ensure our imported translations exist in the file.
+    $this->assertSession()->pageTextContains('msgstr "lundi"');
+
+    // Import some more French translations which will be marked as customized.
+    $name = $file_system->tempnam('temporary://', "po2_") . '.po';
+    file_put_contents($name, $this->getCustomPoFile());
+    $this->drupalGet('admin/config/regional/translate/import');
+    $this->submitForm([
+      'langcode' => 'fr',
+      'files[file]' => $name,
+      'customized' => 1,
+    ], 'Import');
+    $file_system->unlink($name);
+
+    // Create string without translation in the locales_source table.
+    $this->container
+      ->get('locale.storage')
+      ->createString()
+      ->setString('February')
+      ->save();
+
+    // Export only customized French translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm([
+      'langcode' => 'fr',
+      'content_options[not_customized]' => FALSE,
+      'content_options[customized]' => TRUE,
+      'content_options[not_translated]' => FALSE,
+    ], 'Export');
+
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# French translation of Drupal');
+    // Ensure the customized translations exist in the file.
+    $this->assertSession()->pageTextContains('msgstr "janvier"');
+    // Ensure no untranslated strings exist in the file.
+    $this->assertSession()->responseNotContains('msgid "February"');
+
+    // Export only untranslated French translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm([
+      'langcode' => 'fr',
+      'content_options[not_customized]' => FALSE,
+      'content_options[customized]' => FALSE,
+      'content_options[not_translated]' => TRUE,
+    ], 'Export');
+
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# French translation of Drupal');
+    // Ensure no customized translations exist in the file.
+    $this->assertSession()->responseNotContains('msgstr "janvier"');
+    // Ensure the untranslated strings exist in the file, and with right quotes.
+    $this->assertSession()->responseContains($this->getUntranslatedString());
+  }
+
+  /**
+   * Tests exportation of translation template file.
+   */
+  public function testExportTranslationTemplateFile(): void {
+    // Load an admin page with JavaScript so _drupal_add_library() fires at
+    // least once and _locale_parse_js_file() gets to run at least once so that
+    // the locales_source table gets populated with something.
+    $this->drupalGet('admin/config/regional/language');
+    // Get the translation template file.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm([], 'Export');
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# LANGUAGE translation of PROJECT');
+  }
+
+  /**
+   * Helper function that returns a proper .po file.
+   */
+  public function getPoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Monday"
+msgstr "lundi"
+EOF;
+  }
+
+  /**
+   * Returns a .po file that will be marked as customized.
+   */
+  public function getCustomPoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "January"
+msgstr "janvier"
+EOF;
+  }
+
+  /**
+   * Returns a .po file fragment with an untranslated string.
+   *
+   * @return string
+   *   A .po file fragment with an untranslated string.
+   */
+  public function getUntranslatedString() {
+    return <<< EOF
+msgid "February"
+msgstr ""
+EOF;
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleFileSystemFormTest.php

@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the locale functionality in the altered file settings form.
+ *
+ * @group locale
+ */
+class LocaleFileSystemFormTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['system'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $account = $this->drupalCreateUser(['administer site configuration']);
+    $this->drupalLogin($account);
+  }
+
+  /**
+   * Tests translation directory settings on the file settings form.
+   */
+  public function testFileConfigurationPage(): void {
+    // By default there should be no setting for the translation directory.
+    $this->drupalGet('admin/config/media/file-system');
+    $this->assertSession()->fieldNotExists('translation_path');
+
+    // With locale module installed, the setting should appear.
+    $module_installer = $this->container->get('module_installer');
+    $module_installer->install(['locale']);
+    $this->rebuildContainer();
+    $this->drupalGet('admin/config/media/file-system');
+    $this->assertSession()->fieldExists('translation_path');
+
+    // The setting should persist.
+    $translation_path = $this->publicFilesDirectory . '/translations_changed';
+    $fields = [
+      'translation_path' => $translation_path,
+    ];
+    $this->submitForm($fields, 'Save configuration');
+    $this->drupalGet('admin/config/media/file-system');
+    $this->assertSession()->fieldValueEquals('translation_path', $translation_path);
+    $this->assertEquals($this->config('locale.settings')->get('translation.path'), $translation_path);
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleImportFunctionalTest.php

@@ -0,0 +1,679 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\Core\File\FileExists;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore chien chiens deutsch januari lundi moutons műveletek svibanj
+
+/**
+ * Tests the import of locale files.
+ *
+ * @group locale
+ */
+class LocaleImportFunctionalTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'dblog'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * A user able to create languages and import translations.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $adminUser;
+
+  /**
+   * A user able to create languages, import translations, access site reports.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $adminUserAccessSiteReports;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Copy test po files to the translations directory.
+    /** @var \Drupal\Core\File\FileSystemInterface $file_system */
+    $file_system = \Drupal::service('file_system');
+    $file_system->copy(__DIR__ . '/../../../tests/test.de.po', 'translations://', FileExists::Replace);
+    $file_system->copy(__DIR__ . '/../../../tests/test.xx.po', 'translations://', FileExists::Replace);
+
+    $this->adminUser = $this->drupalCreateUser([
+      'administer languages',
+      'translate interface',
+      'access administration pages',
+    ]);
+    $this->adminUserAccessSiteReports = $this->drupalCreateUser([
+      'administer languages',
+      'translate interface',
+      'access administration pages',
+      'access site reports',
+    ]);
+    $this->drupalLogin($this->adminUser);
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+  }
+
+  /**
+   * Tests import of standalone .po files.
+   */
+  public function testStandalonePoFile(): void {
+    // Try importing a .po file.
+    $this->importPoFile($this->getPoFile(), [
+      'langcode' => 'fr',
+    ]);
+    $this->config('locale.settings');
+    // The import should automatically create the corresponding language.
+    $this->assertSession()->pageTextContains("The language French has been created.");
+
+    // The import should have created 8 strings.
+    $this->assertSession()->pageTextContains("One translation file imported. 8 translations were added, 0 translations were updated and 0 translations were removed.");
+
+    // This import should have saved plural forms to have 2 variants.
+    $locale_plurals = \Drupal::service('locale.plural.formula')->getNumberOfPlurals('fr');
+    $this->assertEquals(2, $locale_plurals, 'Plural number initialized.');
+
+    // Ensure we were redirected correctly.
+    $this->assertSession()->addressEquals(Url::fromRoute('locale.translate_page'));
+
+    // Try importing a .po file with invalid tags.
+    $this->importPoFile($this->getBadPoFile(), [
+      'langcode' => 'fr',
+    ]);
+
+    // The import should have created 1 string and rejected 2.
+    $this->assertSession()->pageTextContains("One translation file imported. 1 translations were added, 0 translations were updated and 0 translations were removed.");
+    $this->assertSession()->pageTextContains("2 translation strings were skipped because of disallowed or malformed HTML. See the log for details.");
+
+    // Repeat the process with a user that can access site reports, and this
+    // time the different warnings must contain links to the log.
+    $this->drupalLogin($this->adminUserAccessSiteReports);
+
+    // Try importing a .po file with invalid tags.
+    $this->importPoFile($this->getBadPoFile(), [
+      'langcode' => 'fr',
+    ]);
+
+    $this->assertSession()->pageTextContains("2 translation strings were skipped because of disallowed or malformed HTML. See the log for details.");
+
+    // Check empty files import with a user that cannot access site reports..
+    $this->drupalLogin($this->adminUser);
+    // Try importing a zero byte sized .po file.
+    $this->importPoFile($this->getEmptyPoFile(), [
+      'langcode' => 'fr',
+    ]);
+    // The import should have created 0 string and rejected 0.
+    $this->assertSession()->pageTextContains("One translation file could not be imported. See the log for details.");
+
+    // Repeat the process with a user that can access site reports, and this
+    // time the different warnings must contain links to the log.
+    $this->drupalLogin($this->adminUserAccessSiteReports);
+    // Try importing a zero byte sized .po file.
+    $this->importPoFile($this->getEmptyPoFile(), [
+      'langcode' => 'fr',
+    ]);
+    // The import should have created 0 string and rejected 0.
+    $this->assertSession()->pageTextContains("One translation file could not be imported. See the log for details.");
+
+    // Try importing a .po file which doesn't exist.
+    $name = $this->randomMachineName(16);
+    $this->drupalGet('admin/config/regional/translate/import');
+    $this->submitForm([
+      'langcode' => 'fr',
+      'files[file]' => $name,
+    ], 'Import');
+    $this->assertSession()->addressEquals(Url::fromRoute('locale.translate_import'));
+    $this->assertSession()->pageTextContains('File to import not found.');
+
+    // Try importing a .po file with overriding strings, and ensure existing
+    // strings are kept.
+    $this->importPoFile($this->getOverwritePoFile(), [
+      'langcode' => 'fr',
+    ]);
+
+    // The import should have created 1 string.
+    $this->assertSession()->pageTextContains("One translation file imported. 1 translations were added, 0 translations were updated and 0 translations were removed.");
+    // Ensure string wasn't overwritten.
+    $search = [
+      'string' => 'Montag',
+      'langcode' => 'fr',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // This import should not have changed number of plural forms.
+    $locale_plurals = \Drupal::service('locale.plural.formula')->getNumberOfPlurals('fr');
+    $this->assertEquals(2, $locale_plurals, 'Plural numbers untouched.');
+
+    // Try importing a .po file with overriding strings, and ensure existing
+    // strings are overwritten.
+    $this->importPoFile($this->getOverwritePoFile(), [
+      'langcode' => 'fr',
+      'overwrite_options[not_customized]' => TRUE,
+    ]);
+
+    // The import should have updated 2 strings.
+    $this->assertSession()->pageTextContains("One translation file imported. 0 translations were added, 2 translations were updated and 0 translations were removed.");
+    // Ensure string was overwritten.
+    $search = [
+      'string' => 'Montag',
+      'langcode' => 'fr',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+    // This import should have changed number of plural forms.
+    $locale_plurals = \Drupal::service('locale.plural.formula')->reset()->getNumberOfPlurals('fr');
+    $this->assertEquals(3, $locale_plurals, 'Plural numbers changed.');
+
+    // Importing a .po file and mark its strings as customized strings.
+    $this->importPoFile($this->getCustomPoFile(), [
+      'langcode' => 'fr',
+      'customized' => TRUE,
+    ]);
+
+    // The import should have created 6 strings.
+    $this->assertSession()->pageTextContains("One translation file imported. 6 translations were added, 0 translations were updated and 0 translations were removed.");
+
+    // The database should now contain 6 customized strings (two imported
+    // strings are not translated).
+    $count = Database::getConnection()->select('locales_target')
+      ->condition('customized', 1)
+      ->countQuery()
+      ->execute()
+      ->fetchField();
+    $this->assertEquals(6, $count, 'Customized translations successfully imported.');
+
+    // Try importing a .po file with overriding strings, and ensure existing
+    // customized strings are kept.
+    $this->importPoFile($this->getCustomOverwritePoFile(), [
+      'langcode' => 'fr',
+      'overwrite_options[not_customized]' => TRUE,
+      'overwrite_options[customized]' => FALSE,
+    ]);
+
+    // The import should have created 1 string.
+    $this->assertSession()->pageTextContains("One translation file imported. 1 translations were added, 0 translations were updated and 0 translations were removed.");
+    // Ensure string wasn't overwritten.
+    $search = [
+      'string' => 'januari',
+      'langcode' => 'fr',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Try importing a .po file with overriding strings, and ensure existing
+    // customized strings are overwritten.
+    $this->importPoFile($this->getCustomOverwritePoFile(), [
+      'langcode' => 'fr',
+      'overwrite_options[not_customized]' => FALSE,
+      'overwrite_options[customized]' => TRUE,
+    ]);
+
+    // The import should have updated 2 strings.
+    $this->assertSession()->pageTextContains("One translation file imported. 0 translations were added, 2 translations were updated and 0 translations were removed.");
+    // Ensure string was overwritten.
+    $search = [
+      'string' => 'januari',
+      'langcode' => 'fr',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+
+  }
+
+  /**
+   * Tests msgctxt context support.
+   */
+  public function testLanguageContext(): void {
+    // Try importing a .po file.
+    $this->importPoFile($this->getPoFileWithContext(), [
+      'langcode' => 'hr',
+    ]);
+
+    // We cast the return value of t() to string so as to retrieve the
+    // translated value, rendered as a string.
+    $this->assertSame('Svibanj', (string) t('May', [], ['langcode' => 'hr', 'context' => 'Long month name']), 'Long month name context is working.');
+    $this->assertSame('Svi.', (string) t('May', [], ['langcode' => 'hr']), 'Default context is working.');
+  }
+
+  /**
+   * Tests empty msgstr at end of .po file see #611786.
+   */
+  public function testEmptyMsgstr(): void {
+    $langcode = 'hu';
+
+    // Try importing a .po file.
+    $this->importPoFile($this->getPoFileWithMsgstr(), [
+      'langcode' => $langcode,
+    ]);
+
+    $this->assertSession()->pageTextContains("One translation file imported. 1 translations were added, 0 translations were updated and 0 translations were removed.");
+    $this->assertSame('Műveletek', (string) t('Operations', [], ['langcode' => $langcode]), 'String imported and translated.');
+
+    // Try importing a .po file.
+    $this->importPoFile($this->getPoFileWithEmptyMsgstr(), [
+      'langcode' => $langcode,
+      'overwrite_options[not_customized]' => TRUE,
+    ]);
+    $this->assertSession()->pageTextContains("One translation file imported. 0 translations were added, 0 translations were updated and 1 translations were removed.");
+
+    $str = "Operations";
+    $search = [
+      'string' => $str,
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    // Check that search finds the string as untranslated.
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($str);
+  }
+
+  /**
+   * Tests .po file import with configuration translation.
+   */
+  public function testConfigPoFile(): void {
+    // Values for translations to assert. Config key, original string,
+    // translation and config property name.
+    $config_strings = [
+      'system.maintenance' => [
+        '@site is currently under maintenance. We should be back shortly. Thank you for your patience.',
+        // cSpell:disable-next-line
+        '@site karbantartás alatt áll. Rövidesen visszatérünk. Köszönjük a türelmet.',
+        'message',
+      ],
+      'user.role.anonymous' => [
+        'Anonymous user',
+        // cSpell:disable-next-line
+        'Névtelen felhasználó',
+        'label',
+      ],
+    ];
+
+    // Add custom language for testing.
+    $langcode = 'xx';
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $this->randomMachineName(16),
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Check for the source strings we are going to translate. Adding the
+    // custom language should have made the process to export configuration
+    // strings to interface translation executed.
+    $locale_storage = $this->container->get('locale.storage');
+    foreach ($config_strings as $config_string) {
+      $string = $locale_storage->findString(['source' => $config_string[0], 'context' => '', 'type' => 'configuration']);
+      $this->assertNotEmpty($string, 'Configuration strings have been created upon installation.');
+    }
+
+    // Import a .po file to translate.
+    $this->importPoFile($this->getPoFileWithConfig(), [
+      'langcode' => $langcode,
+    ]);
+
+    // Translations got recorded in the interface translation system.
+    foreach ($config_strings as $config_string) {
+      $search = [
+        'string' => $config_string[0],
+        'langcode' => $langcode,
+        'translation' => 'all',
+      ];
+      $this->drupalGet('admin/config/regional/translate');
+      $this->submitForm($search, 'Filter');
+      $this->assertSession()->pageTextContains($config_string[1]);
+    }
+
+    // Test that translations got recorded in the config system.
+    $overrides = \Drupal::service('language.config_factory_override');
+    foreach ($config_strings as $config_key => $config_string) {
+      $override = $overrides->getOverride($langcode, $config_key);
+      $this->assertEquals($override->get($config_string[2]), $config_string[1]);
+    }
+  }
+
+  /**
+   * Tests .po file import with user.settings configuration.
+   */
+  public function testConfigTranslationImportingPoFile(): void {
+    // Set the language code.
+    $langcode = 'de';
+
+    // Import a .po file to translate.
+    $this->importPoFile($this->getPoFileWithConfigDe(), [
+      'langcode' => $langcode,
+    ]);
+
+    // Check that the 'Anonymous' string is translated.
+    $config = \Drupal::languageManager()->getLanguageConfigOverride($langcode, 'user.settings');
+    $this->assertEquals('Anonymous German', $config->get('anonymous'));
+  }
+
+  /**
+   * Tests the translation are imported when a new language is created.
+   */
+  public function testCreatedLanguageTranslation(): void {
+    // Import a .po file to add de language.
+    $this->importPoFile($this->getPoFileWithConfigDe(), ['langcode' => 'de']);
+
+    // Get the language.entity.de label and check it's been translated.
+    $override = \Drupal::languageManager()->getLanguageConfigOverride('de', 'language.entity.de');
+    $this->assertEquals('Deutsch', $override->get('label'));
+  }
+
+  /**
+   * Helper function: import a standalone .po file in a given language.
+   *
+   * @param string $contents
+   *   Contents of the .po file to import.
+   * @param array $options
+   *   (optional) Additional options to pass to the translation import form.
+   */
+  public function importPoFile($contents, array $options = []) {
+    $file_system = \Drupal::service('file_system');
+    $name = $file_system->tempnam('temporary://', "po_") . '.po';
+    file_put_contents($name, $contents);
+    $options['files[file]'] = $name;
+    $this->drupalGet('admin/config/regional/translate/import');
+    $this->submitForm($options, 'Import');
+    $file_system->unlink($name);
+  }
+
+  /**
+   * Helper function that returns a proper .po file.
+   */
+  public function getPoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "One sheep"
+msgid_plural "@count sheep"
+msgstr[0] "un mouton"
+msgstr[1] "@count moutons"
+
+msgid "Monday"
+msgstr "lundi"
+
+msgid "Tuesday"
+msgstr "mardi"
+
+msgid "Wednesday"
+msgstr "mercredi"
+
+msgid "Thursday"
+msgstr "jeudi"
+
+msgid "Friday"
+msgstr "vendredi"
+
+msgid "Saturday"
+msgstr "samedi"
+
+msgid "Sunday"
+msgstr "dimanche"
+EOF;
+  }
+
+  /**
+   * Helper function that returns an empty .po file.
+   */
+  public function getEmptyPoFile() {
+    return '';
+  }
+
+  /**
+   * Helper function that returns a bad .po file.
+   */
+  public function getBadPoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Save configuration"
+msgstr "Enregistrer la configuration"
+
+msgid "edit"
+msgstr "modifier<img SRC="javascript:alert(\'xss\');">"
+
+msgid "delete"
+msgstr "supprimer<script>alert('xss');</script>"
+
+EOF;
+  }
+
+  /**
+   * Helper function that returns a proper .po file for testing.
+   */
+  public function getOverwritePoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;\\n"
+
+msgid "Monday"
+msgstr "Montag"
+
+msgid "Day"
+msgstr "Jour"
+EOF;
+  }
+
+  /**
+   * Returns a .po file that will be marked as customized.
+   */
+  public function getCustomPoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "One dog"
+msgid_plural "@count dogs"
+msgstr[0] "un chien"
+msgstr[1] "@count chiens"
+
+msgid "January"
+msgstr "janvier"
+
+msgid "February"
+msgstr "février"
+
+msgid "March"
+msgstr "mars"
+
+msgid "April"
+msgstr "avril"
+
+msgid "June"
+msgstr "juin"
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file for testing customized strings.
+   */
+  public function getCustomOverwritePoFile() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "January"
+msgstr "januari"
+
+msgid "February"
+msgstr "februari"
+
+msgid "July"
+msgstr "juillet"
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file with context.
+   */
+  public function getPoFileWithContext() {
+    // Croatian (code hr) is one of the languages that have a different
+    // form for the full name and the abbreviated name for the month of May.
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;\\n"
+
+msgctxt "Long month name"
+msgid "May"
+msgstr "Svibanj"
+
+msgid "May"
+msgstr "Svi."
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file with an empty last item.
+   */
+  public function getPoFileWithEmptyMsgstr() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Operations"
+msgstr ""
+
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file with an empty last item.
+   */
+  public function getPoFileWithMsgstr() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Operations"
+msgstr "Műveletek"
+
+msgid "Will not appear in Drupal core, so we can ensure the test passes"
+msgstr ""
+
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file with configuration translations.
+   */
+  public function getPoFileWithConfig() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "@site is currently under maintenance. We should be back shortly. Thank you for your patience."
+msgstr "@site karbantartás alatt áll. Rövidesen visszatérünk. Köszönjük a türelmet."
+
+msgid "Anonymous user"
+msgstr "Névtelen felhasználó"
+
+EOF;
+  }
+
+  /**
+   * Helper function that returns a .po file with configuration translations.
+   */
+  public function getPoFileWithConfigDe() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Anonymous"
+msgstr "Anonymous German"
+
+msgid "German"
+msgstr "Deutsch"
+
+EOF;
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleInstallTest.php

@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Test installation of Locale module.
+ *
+ * @group locale
+ */
+class LocaleInstallTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'system',
+    'file',
+    'language',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests Locale install message.
+   */
+  public function testLocaleInstallMessage(): void {
+    $admin_user = $this->drupalCreateUser([
+      'access administration pages',
+      'administer modules',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    $edit = [];
+    $edit['modules[locale][enable]'] = 'locale';
+    $this->drupalGet('admin/modules');
+    $this->submitForm($edit, 'Install');
+
+    $this->assertSession()->statusMessageContains('available translations', 'status');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleJavascriptTranslationTest.php

@@ -0,0 +1,173 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Component\Gettext\PoItem;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore descripcion mostrar
+
+/**
+ * Tests parsing js files for translatable strings.
+ *
+ * @group locale
+ */
+class LocaleJavascriptTranslationTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'locale_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  public function testFileParsing(): void {
+
+    // This test is for ensuring that the regular expression in
+    // _locale_parse_js_file() finds translatable source strings in all valid
+    // JavaScript syntax regardless of the coding style used, especially with
+    // respect to optional whitespace, line breaks, etc.
+    // - We test locale_test.js, because that is the one that contains a
+    //   variety of whitespace styles.
+    $files[] = __DIR__ . '/../../locale_test.js';
+
+    foreach ($files as $filename) {
+      // Parse the file to look for source strings.
+      _locale_parse_js_file($filename);
+
+      // Get all of the source strings that were found.
+      $strings = $this->container
+        ->get('locale.storage')
+        ->getStrings([
+          'type' => 'javascript',
+          'name' => $filename,
+        ]);
+
+      $source_strings = [];
+      foreach ($strings as $string) {
+        $source_strings[$string->source] = $string->context;
+      }
+
+      $etx = PoItem::DELIMITER;
+      // List of all strings that should be in the file.
+      $test_strings = [
+        'Standard Call t' => '',
+        'Whitespace Call t' => '',
+
+        'Single Quote t' => '',
+        "Single Quote \\'Escaped\\' t" => '',
+        'Single Quote Concat strings t' => '',
+
+        'Double Quote t' => '',
+        "Double Quote \\\"Escaped\\\" t" => '',
+        'Double Quote Concat strings t' => '',
+
+        'Context !key Args t' => 'Context string',
+
+        'Context Unquoted t' => 'Context string unquoted',
+        'Context Single Quoted t' => 'Context string single quoted',
+        'Context Double Quoted t' => 'Context string double quoted',
+
+        "Standard Call plural{$etx}Standard Call @count plural" => '',
+        "Whitespace Call plural{$etx}Whitespace Call @count plural" => '',
+
+        "Single Quote plural{$etx}Single Quote @count plural" => '',
+        "Single Quote \\'Escaped\\' plural{$etx}Single Quote \\'Escaped\\' @count plural" => '',
+
+        "Double Quote plural{$etx}Double Quote @count plural" => '',
+        "Double Quote \\\"Escaped\\\" plural{$etx}Double Quote \\\"Escaped\\\" @count plural" => '',
+
+        "Context !key Args plural{$etx}Context !key Args @count plural" => 'Context string',
+
+        "Context Unquoted plural{$etx}Context Unquoted @count plural" => 'Context string unquoted',
+        "Context Single Quoted plural{$etx}Context Single Quoted @count plural" => 'Context string single quoted',
+        "Context Double Quoted plural{$etx}Context Double Quoted @count plural" => 'Context string double quoted',
+
+        "No count argument plural - singular{$etx}No count argument plural - plural" => '',
+      ];
+
+      // Assert that all strings were found properly.
+      foreach ($test_strings as $str => $context) {
+
+        // Make sure that the string was found in the file.
+        $this->assertTrue(isset($source_strings[$str]), "Found source string: $str");
+
+        // Make sure that the proper context was matched.
+        $this->assertArrayHasKey($str, $source_strings);
+        $this->assertSame($context, $source_strings[$str]);
+      }
+
+      $this->assertSameSize($test_strings, $source_strings, 'Found correct number of source strings.');
+    }
+  }
+
+  /**
+   * Assert translations JS is added before drupal.js, because it depends on it.
+   */
+  public function testLocaleTranslationJsDependencies(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+
+    // Add custom language.
+    $this->drupalLogin($admin_user);
+    // Code for the language.
+    $langcode = 'es';
+    // The English name for the language.
+    $name = $this->randomMachineName(16);
+    // The domain prefix.
+    $prefix = $langcode;
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Set path prefix.
+    $edit = ["prefix[$langcode]" => $prefix];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+
+    // This forces locale.admin.js string sources to be imported, which contains
+    // the next translation.
+    $this->drupalGet($prefix . '/admin/config/regional/translate');
+
+    // Translate a string in locale.admin.js to our new language.
+    $strings = \Drupal::service('locale.storage')
+      ->getStrings([
+        'source' => 'Show description',
+        'type' => 'javascript',
+        'name' => 'core/modules/locale/locale.admin.js',
+      ]);
+    $string = $strings[0];
+
+    $this->submitForm(['string' => 'Show description'], 'Filter');
+    $edit = ['strings[' . $string->lid . '][translations][0]' => 'Mostrar descripcion'];
+    $this->submitForm($edit, 'Save translations');
+
+    // Calculate the filename of the JS including the translations.
+    $js_translation_files = \Drupal::state()->get('locale.translation.javascript');
+    $js_filename = $prefix . '_' . $js_translation_files[$prefix] . '.js';
+
+    $content = $this->getSession()->getPage()->getContent();
+    $this->assertSession()->responseContains('core/misc/drupal.js');
+    $this->assertSession()->responseContains($js_filename);
+    // Assert translations JS is included before drupal.js.
+    $this->assertLessThan(strpos($content, 'core/misc/drupal.js'), strpos($content, $js_filename));
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleLocaleLookupTest.php

@@ -0,0 +1,119 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Component\Gettext\PoItem;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\WaitTerminateTestTrait;
+
+/**
+ * Tests LocaleLookup.
+ *
+ * @group locale
+ */
+class LocaleLocaleLookupTest extends BrowserTestBase {
+
+  use WaitTerminateTestTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'locale_test'];
+
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // The \Drupal\locale\LocaleTranslation service stores localization cache
+    // data after the response is flushed to the client. We do not want to race
+    // with any string translations that may be saving from the login below.
+    $this->setWaitForTerminate();
+
+    // Change the language default object to different values.
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+    $this->config('system.site')->set('default_langcode', 'fr')->save();
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'administer modules',
+    ]));
+  }
+
+  /**
+   * Tests that there are no circular dependencies.
+   */
+  public function testCircularDependency(): void {
+    // Ensure that we can enable early_translation_test on a non-english site.
+    $this->drupalGet('admin/modules');
+    $this->submitForm(['modules[early_translation_test][enable]' => TRUE], 'Install');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
+  /**
+   * Tests language fallback defaults.
+   */
+  public function testLanguageFallbackDefaults(): void {
+    $this->drupalGet('');
+    // Ensure state of fallback languages persisted by
+    // locale_test_language_fallback_candidates_locale_lookup_alter() is empty.
+    $this->assertEquals([], \Drupal::state()->get('locale.test_language_fallback_candidates_locale_lookup_alter_candidates'));
+    // Make sure there is enough information provided for alter hooks.
+    $context = \Drupal::state()->get('locale.test_language_fallback_candidates_locale_lookup_alter_context');
+    $this->assertEquals('fr', $context['langcode']);
+    $this->assertEquals('locale_lookup', $context['operation']);
+  }
+
+  /**
+   * Tests old plural style @count[number] fix.
+   *
+   * @dataProvider providerTestFixOldPluralStyle
+   */
+  public function testFixOldPluralStyle($translation_value, $expected): void {
+    $string_storage = \Drupal::service('locale.storage');
+    $string = $string_storage->findString(['source' => 'Member for', 'context' => '']);
+    $lid = $string->getId();
+    $string_storage->createTranslation([
+      'lid' => $lid,
+      'language' => 'fr',
+      'translation' => $translation_value,
+    ])->save();
+    _locale_refresh_translations(['fr'], [$lid]);
+
+    // Check that 'count[2]' was fixed for render value.
+    $this->drupalGet('');
+    $this->assertSession()->pageTextContains($expected);
+
+    // Check that 'count[2]' was saved for source value.
+    $translation = $string_storage->findTranslation(['language' => 'fr', 'lid' => $lid])->translation;
+    $this->assertSame($translation_value, $translation, 'Source value not changed');
+    $this->assertStringContainsString('@count[2]', $translation, 'Source value contains @count[2]');
+  }
+
+  /**
+   * Provides data for testFixOldPluralStyle().
+   *
+   * @return array
+   *   An array of test data:
+   *     - translation value
+   *     - expected result
+   */
+  public static function providerTestFixOldPluralStyle() {
+    return [
+      'non-plural translation' => ['@count[2] non-plural test', '@count[2] non-plural test'],
+      'plural translation' => ['@count[2] plural test' . PoItem::DELIMITER, '@count plural test'],
+    ];
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleNonInteractiveDevInstallTest.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+/**
+ * Tests installing in a different language with a dev version string.
+ *
+ * @group locale
+ */
+class LocaleNonInteractiveDevInstallTest extends LocaleNonInteractiveInstallTest {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getVersionStringToTest() {
+    include_once $this->root . '/core/includes/install.core.inc';
+    $version = _install_get_version_info(\Drupal::VERSION);
+    return $version['major'] . '.' . $version['minor'] . '.x';
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleNonInteractiveInstallTest.php

@@ -0,0 +1,73 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests installing in a different language with a non-dev version string.
+ *
+ * @group locale
+ */
+class LocaleNonInteractiveInstallTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    $this->markTestSkipped('Skipped due to major version-specific logic. See https://www.drupal.org/project/drupal/issues/3359322');
+    parent::setUp();
+  }
+
+  /**
+   * Gets the version string to use in the translation file.
+   *
+   * @return string
+   *   The version string to test, for example, '8.0.0' or '8.6.x'.
+   */
+  protected function getVersionStringToTest() {
+    include_once $this->root . '/core/includes/install.core.inc';
+    $version = _install_get_version_info(\Drupal::VERSION);
+    return $version['major'] . '.0.0';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function installParameters() {
+    $parameters = parent::installParameters();
+    // Install Drupal in German.
+    $parameters['parameters']['langcode'] = 'de';
+    // Create a po file so we don't attempt to download one from
+    // localize.drupal.org and to have a test translation that will not change.
+    \Drupal::service('file_system')->mkdir($this->publicFilesDirectory . '/translations', NULL, TRUE);
+    $contents = <<<PO
+msgid ""
+msgstr ""
+
+msgid "Enter the password that accompanies your username."
+msgstr "Geben sie das Passwort für ihren Benutzernamen ein."
+
+PO;
+    $version = $this->getVersionStringToTest();
+    file_put_contents($this->publicFilesDirectory . "/translations/drupal-{$version}.de.po", $contents);
+    return $parameters;
+  }
+
+  /**
+   * Tests that the expected translated text appears on the login screen.
+   */
+  public function testInstallerTranslations(): void {
+    $this->drupalGet('user/login');
+    // cSpell:disable-next-line
+    $this->assertSession()->responseContains('Geben sie das Passwort für ihren Benutzernamen ein.');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocalePathTest.php

@@ -0,0 +1,162 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\Traits\Core\PathAliasTestTrait;
+
+/**
+ * Tests you can configure a language for individual URL aliases.
+ *
+ * @group locale
+ * @group path
+ */
+class LocalePathTest extends BrowserTestBase {
+
+  use PathAliasTestTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['node', 'locale', 'path', 'views'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->drupalCreateContentType(['type' => 'page', 'name' => 'Basic page']);
+    $this->config('system.site')->set('page.front', '/node')->save();
+  }
+
+  /**
+   * Tests if a language can be associated with a path alias.
+   */
+  public function testPathLanguageConfiguration(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'create page content',
+      'administer url aliases',
+      'create url aliases',
+      'access administration pages',
+      'access content overview',
+    ]);
+
+    // Add custom language.
+    $this->drupalLogin($admin_user);
+    // Code for the language.
+    $langcode = 'xx';
+    // The English name for the language.
+    $name = $this->randomMachineName(16);
+    // The domain prefix.
+    $prefix = $langcode;
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Set path prefix.
+    $edit = ["prefix[$langcode]" => $prefix];
+    $this->drupalGet('admin/config/regional/language/detection/url');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Check that the "xx" front page is readily available because path prefix
+    // negotiation is pre-configured.
+    $this->drupalGet($prefix);
+    $this->assertSession()->pageTextContains('Welcome!');
+
+    // Create a node.
+    $node = $this->drupalCreateNode(['type' => 'page']);
+
+    // Create a path alias in default language (English).
+    $path = 'admin/config/search/path/add';
+    $english_path = $this->randomMachineName(8);
+    $edit = [
+      'path[0][value]' => '/node/' . $node->id(),
+      'alias[0][value]' => '/' . $english_path,
+      'langcode[0][value]' => 'en',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save');
+
+    // Create a path alias in new custom language.
+    $custom_language_path = $this->randomMachineName(8);
+    $edit = [
+      'path[0][value]' => '/node/' . $node->id(),
+      'alias[0][value]' => '/' . $custom_language_path,
+      'langcode[0][value]' => $langcode,
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save');
+
+    // Confirm English language path alias works.
+    $this->drupalGet($english_path);
+    $this->assertSession()->pageTextContains($node->label());
+
+    // Confirm custom language path alias works.
+    $this->drupalGet($prefix . '/' . $custom_language_path);
+    $this->assertSession()->pageTextContains($node->label());
+
+    // Create a custom path.
+    $custom_path = $this->randomMachineName(8);
+
+    // Check priority of language for alias by source path.
+    $path_alias = $this->createPathAlias('/node/' . $node->id(), '/' . $custom_path, LanguageInterface::LANGCODE_NOT_SPECIFIED);
+    $lookup_path = $this->container->get('path_alias.manager')->getAliasByPath('/node/' . $node->id(), 'en');
+    $this->assertEquals('/' . $english_path, $lookup_path, 'English language alias has priority.');
+    // Same check for language 'xx'.
+    $lookup_path = $this->container->get('path_alias.manager')->getAliasByPath('/node/' . $node->id(), $prefix);
+    $this->assertEquals('/' . $custom_language_path, $lookup_path, 'Custom language alias has priority.');
+    $path_alias->delete();
+
+    // Create language nodes to check priority of aliases.
+    $first_node = $this->drupalCreateNode(['type' => 'page', 'promote' => 1, 'langcode' => 'en']);
+    $second_node = $this->drupalCreateNode(['type' => 'page', 'promote' => 1, 'langcode' => LanguageInterface::LANGCODE_NOT_SPECIFIED]);
+
+    // Assign a custom path alias to the first node with the English language.
+    $this->createPathAlias('/node/' . $first_node->id(), '/' . $custom_path, $first_node->language()->getId());
+
+    // Assign a custom path alias to second node with
+    // LanguageInterface::LANGCODE_NOT_SPECIFIED.
+    $this->createPathAlias('/node/' . $second_node->id(), '/' . $custom_path, $second_node->language()->getId());
+
+    // Test that both node titles link to our path alias.
+    $this->drupalGet('admin/content');
+    $custom_path_url = Url::fromUserInput('/' . $custom_path)->toString();
+    $this->assertSession()->elementExists('xpath', $this->assertSession()->buildXPathQuery('//a[@href=:href and normalize-space(text())=:title]', [
+      ':href' => $custom_path_url,
+      ':title' => $first_node->label(),
+    ]));
+    $this->assertSession()->elementExists('xpath', $this->assertSession()->buildXPathQuery('//a[@href=:href and normalize-space(text())=:title]', [
+      ':href' => $custom_path_url,
+      ':title' => $second_node->label(),
+    ]));
+
+    // Confirm that the custom path leads to the first node.
+    $this->drupalGet($custom_path);
+    $this->assertSession()->pageTextContains($first_node->label());
+
+    // Confirm that the custom path with prefix leads to the second node.
+    $this->drupalGet($prefix . '/' . $custom_path);
+    $this->assertSession()->pageTextContains($second_node->label());
+
+  }
+
+}

core/modules/locale/tests/src/Functional/LocalePluralFormatTest.php

@@ -0,0 +1,487 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Component\Gettext\PoItem;
+use Drupal\Core\Database\Database;
+use Drupal\Core\StringTranslation\PluralTranslatableMarkup;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore heure heures jours lundi ponedjeljak
+
+/**
+ * Tests plural handling for various languages.
+ *
+ * @group locale
+ */
+class LocalePluralFormatTest extends BrowserTestBase {
+
+  /**
+   * An admin user.
+   *
+   * @var \Drupal\user\Entity\User
+   */
+  protected $adminUser;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->adminUser = $this->drupalCreateUser([
+      'administer languages',
+      'translate interface',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($this->adminUser);
+  }
+
+  /**
+   * Tests locale_get_plural() and \Drupal::translation()->formatPlural().
+   */
+  public function testGetPluralFormat(): void {
+    // Import some .po files with formulas to set up the environment.
+    // These will also add the languages to the system.
+    $this->importPoFile($this->getPoFileWithSimplePlural(), [
+      'langcode' => 'fr',
+    ]);
+    $this->importPoFile($this->getPoFileWithComplexPlural(), [
+      'langcode' => 'hr',
+    ]);
+
+    // Attempt to import some broken .po files as well to prove that these
+    // will not overwrite the proper plural formula imported above.
+    $this->importPoFile($this->getPoFileWithMissingPlural(), [
+      'langcode' => 'fr',
+      'overwrite_options[not_customized]' => TRUE,
+    ]);
+    $this->importPoFile($this->getPoFileWithBrokenPlural(), [
+      'langcode' => 'hr',
+      'overwrite_options[not_customized]' => TRUE,
+    ]);
+
+    // Reset static caches from locale_get_plural() to ensure we get fresh data.
+    drupal_static_reset('locale_get_plural');
+    drupal_static_reset('locale_get_plural:plurals');
+    drupal_static_reset('locale');
+
+    // Expected plural translation strings for each plural index.
+    $plural_strings = [
+      // English is not imported in this case, so we assume built-in text
+      // and formulas.
+      'en' => [
+        0 => '1 hour',
+        1 => '@count hours',
+      ],
+      'fr' => [
+        0 => '@count heure',
+        1 => '@count heures',
+      ],
+      'hr' => [
+        0 => '@count sat',
+        1 => '@count sata',
+        2 => '@count sati',
+      ],
+      // Hungarian is not imported, so it should assume the same text as
+      // English, but it will always pick the plural form as per the built-in
+      // logic, so only index -1 is relevant with the plural value.
+      'hu' => [
+        0 => '1 hour',
+        -1 => '@count hours',
+      ],
+    ];
+
+    // Expected plural indexes precomputed base on the plural formulas with
+    // given $count value.
+    $plural_tests = [
+      'en' => [
+        1 => 0,
+        0 => 1,
+        5 => 1,
+        123 => 1,
+        235 => 1,
+      ],
+      'fr' => [
+        1 => 0,
+        0 => 0,
+        5 => 1,
+        123 => 1,
+        235 => 1,
+      ],
+      'hr' => [
+        1 => 0,
+        21 => 0,
+        0 => 2,
+        2 => 1,
+        8 => 2,
+        123 => 1,
+        235 => 2,
+      ],
+      'hu' => [
+        1 => -1,
+        21 => -1,
+        0 => -1,
+      ],
+    ];
+
+    foreach ($plural_tests as $langcode => $tests) {
+      foreach ($tests as $count => $expected_plural_index) {
+        // Assert that the we get the right plural index.
+        $this->assertSame($expected_plural_index, locale_get_plural($count, $langcode), 'Computed plural index for ' . $langcode . ' for count ' . $count . ' is ' . $expected_plural_index);
+        // Assert that the we get the right translation for that. Change the
+        // expected index as per the logic for translation lookups.
+        $expected_plural_index = ($count == 1) ? 0 : $expected_plural_index;
+        $expected_plural_string = str_replace('@count', (string) $count, $plural_strings[$langcode][$expected_plural_index]);
+        $this->assertSame($expected_plural_string, \Drupal::translation()->formatPlural($count, '@count hour', '@count hours', [], ['langcode' => $langcode])->render(), 'Plural translation of @count hour / @count hours for count ' . $count . ' in ' . $langcode . ' is ' . $expected_plural_string);
+        // DO NOT use translation to pass translated strings into
+        // PluralTranslatableMarkup::createFromTranslatedString() this way. It
+        // is designed to be used with *already* translated text like settings
+        // from configuration. We use PHP translation here just because we have
+        // the expected result data in that format.
+        $translated_string = \Drupal::translation()->translate('@count hour' . PoItem::DELIMITER . '@count hours', [], ['langcode' => $langcode]);
+        $plural = PluralTranslatableMarkup::createFromTranslatedString($count, $translated_string, [], ['langcode' => $langcode]);
+        $this->assertSame($expected_plural_string, $plural->render());
+      }
+    }
+  }
+
+  /**
+   * Tests plural editing of DateFormatter strings.
+   */
+  public function testPluralEditDateFormatter(): void {
+
+    // Import some .po files with formulas to set up the environment.
+    // These will also add the languages to the system.
+    $this->importPoFile($this->getPoFileWithSimplePlural(), [
+      'langcode' => 'fr',
+    ]);
+
+    // Set French as the site default language.
+    $this->config('system.site')->set('default_langcode', 'fr')->save();
+
+    // Visit User Info page before updating translation strings. Change the
+    // created time to ensure that the we're dealing in seconds and it can't be
+    // exactly 1 minute.
+    $this->adminUser->set('created', time() - 1)->save();
+    $this->drupalGet('user');
+
+    // Member for time should be translated.
+    $this->assertSession()->pageTextContains("seconde");
+
+    $path = 'admin/config/regional/translate/';
+    $search = [
+      'langcode' => 'fr',
+      // Limit to only translated strings to ensure that database ordering does
+      // not break the test.
+      'translation' => 'translated',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($search, 'Filter');
+    // Plural values for the langcode fr.
+    $this->assertSession()->pageTextContains('@count seconde');
+    $this->assertSession()->pageTextContains('@count secondes');
+
+    // Inject a plural source string to the database. We need to use a specific
+    // langcode here because the language will be English by default and will
+    // not save our source string for performance optimization if we do not ask
+    // specifically for a language.
+    \Drupal::translation()->formatPlural(1, '@count second', '@count seconds', [], ['langcode' => 'fr'])->render();
+    $lid = Database::getConnection()->select('locales_source', 'ls')
+      ->fields('ls', ['lid'])
+      ->condition('source', "@count second" . PoItem::DELIMITER . "@count seconds")
+      ->condition('context', '')
+      ->execute()
+      ->fetchField();
+    // Look up editing page for this plural string and check fields.
+    $search = [
+      'string' => '@count second',
+      'langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+
+    // Save complete translations for the string in langcode fr.
+    $edit = [
+      "strings[$lid][translations][0]" => '@count seconde updated',
+      "strings[$lid][translations][1]" => '@count secondes updated',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save translations');
+
+    // User interface input for translating seconds should not be duplicated
+    $this->assertSession()->pageTextContainsOnce('@count seconds');
+
+    // Member for time should be translated. Change the created time to ensure
+    // that the we're dealing in multiple seconds and it can't be exactly 1
+    // second or minute.
+    $this->adminUser->set('created', time() - 2)->save();
+    $this->drupalGet('user');
+    $this->assertSession()->pageTextContains("secondes updated");
+  }
+
+  /**
+   * Tests plural editing and export functionality.
+   */
+  public function testPluralEditExport(): void {
+    // Import some .po files with formulas to set up the environment.
+    // These will also add the languages to the system.
+    $this->importPoFile($this->getPoFileWithSimplePlural(), [
+      'langcode' => 'fr',
+    ]);
+    $this->importPoFile($this->getPoFileWithComplexPlural(), [
+      'langcode' => 'hr',
+    ]);
+
+    // Get the French translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm(['langcode' => 'fr'], 'Export');
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# French translation of Drupal');
+    // Ensure our imported translations exist in the file.
+    $this->assertSession()->responseContains("msgid \"Monday\"\nmsgstr \"lundi\"");
+    // Check for plural export specifically.
+    $this->assertSession()->responseContains("msgid \"@count hour\"\nmsgid_plural \"@count hours\"\nmsgstr[0] \"@count heure\"\nmsgstr[1] \"@count heures\"");
+
+    // Get the Croatian translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm(['langcode' => 'hr'], 'Export');
+    // Ensure we have a translation file.
+    $this->assertSession()->pageTextContains('# Croatian translation of Drupal');
+    // Ensure our imported translations exist in the file.
+    $this->assertSession()->responseContains("msgid \"Monday\"\nmsgstr \"Ponedjeljak\"");
+    // Check for plural export specifically.
+    $this->assertSession()->responseContains("msgid \"@count hour\"\nmsgid_plural \"@count hours\"\nmsgstr[0] \"@count sat\"\nmsgstr[1] \"@count sata\"\nmsgstr[2] \"@count sati\"");
+
+    // Check if the source appears on the translation page.
+    $this->drupalGet('admin/config/regional/translate');
+    $this->assertSession()->pageTextContains("@count hour");
+    $this->assertSession()->pageTextContains("@count hours");
+
+    // Look up editing page for this plural string and check fields.
+    $path = 'admin/config/regional/translate/';
+    $search = [
+      'langcode' => 'hr',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($search, 'Filter');
+    // Labels for plural editing elements.
+    $this->assertSession()->pageTextContains('Singular form');
+    $this->assertSession()->pageTextContains('First plural form');
+    $this->assertSession()->pageTextContains('2. plural form');
+    $this->assertSession()->pageTextNotContains('3. plural form');
+
+    // Plural values for langcode hr.
+    $this->assertSession()->pageTextContains('@count sat');
+    $this->assertSession()->pageTextContains('@count sata');
+    $this->assertSession()->pageTextContains('@count sati');
+
+    $connection = Database::getConnection();
+    // Edit langcode hr translations and see if that took effect.
+    $lid = $connection->select('locales_source', 'ls')
+      ->fields('ls', ['lid'])
+      ->condition('source', "@count hour" . PoItem::DELIMITER . "@count hours")
+      ->condition('context', '')
+      ->execute()
+      ->fetchField();
+    $edit = [
+      "strings[$lid][translations][1]" => '@count sata edited',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save translations');
+
+    $search = [
+      'langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    // Plural values for the langcode fr.
+    $this->assertSession()->pageTextContains('@count heure');
+    $this->assertSession()->pageTextContains('@count heures');
+    $this->assertSession()->pageTextNotContains('2. plural form');
+
+    // Edit langcode fr translations and see if that took effect.
+    $edit = [
+      "strings[$lid][translations][0]" => '@count heure edited',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save translations');
+
+    // Inject a plural source string to the database. We need to use a specific
+    // langcode here because the language will be English by default and will
+    // not save our source string for performance optimization if we do not ask
+    // specifically for a language.
+    \Drupal::translation()->formatPlural(1, '@count day', '@count days', [], ['langcode' => 'fr'])->render();
+    $lid = $connection->select('locales_source', 'ls')
+      ->fields('ls', ['lid'])
+      ->condition('source', "@count day" . PoItem::DELIMITER . "@count days")
+      ->condition('context', '')
+      ->execute()
+      ->fetchField();
+    // Look up editing page for this plural string and check fields.
+    $search = [
+      'string' => '@count day',
+      'langcode' => 'fr',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+
+    // Save complete translations for the string in langcode fr.
+    $edit = [
+      "strings[$lid][translations][0]" => '@count jour',
+      "strings[$lid][translations][1]" => '@count jours',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save translations');
+
+    // Save complete translations for the string in langcode hr.
+    $search = [
+      'string' => '@count day',
+      'langcode' => 'hr',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+
+    $edit = [
+      "strings[$lid][translations][0]" => '@count dan',
+      "strings[$lid][translations][1]" => '@count dana',
+      "strings[$lid][translations][2]" => '@count dana',
+    ];
+    $this->drupalGet($path);
+    $this->submitForm($edit, 'Save translations');
+
+    // Get the French translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm(['langcode' => 'fr'], 'Export');
+    // Check for plural export specifically.
+    $this->assertSession()->responseContains("msgid \"@count hour\"\nmsgid_plural \"@count hours\"\nmsgstr[0] \"@count heure edited\"\nmsgstr[1] \"@count heures\"");
+    $this->assertSession()->responseContains("msgid \"@count day\"\nmsgid_plural \"@count days\"\nmsgstr[0] \"@count jour\"\nmsgstr[1] \"@count jours\"");
+
+    // Get the Croatian translations.
+    $this->drupalGet('admin/config/regional/translate/export');
+    $this->submitForm(['langcode' => 'hr'], 'Export');
+    // Check for plural export specifically.
+    $this->assertSession()->responseContains("msgid \"@count hour\"\nmsgid_plural \"@count hours\"\nmsgstr[0] \"@count sat\"\nmsgstr[1] \"@count sata edited\"\nmsgstr[2] \"@count sati\"");
+    $this->assertSession()->responseContains("msgid \"@count day\"\nmsgid_plural \"@count days\"\nmsgstr[0] \"@count dan\"\nmsgstr[1] \"@count dana\"\nmsgstr[2] \"@count dana\"");
+  }
+
+  /**
+   * Imports a standalone .po file in a given language.
+   *
+   * @param string $contents
+   *   Contents of the .po file to import.
+   * @param array $options
+   *   Additional options to pass to the translation import form.
+   */
+  public function importPoFile($contents, array $options = []) {
+    $file_system = \Drupal::service('file_system');
+    $name = $file_system->tempnam('temporary://', "po_") . '.po';
+    file_put_contents($name, $contents);
+    $options['files[file]'] = $name;
+    $this->drupalGet('admin/config/regional/translate/import');
+    $this->submitForm($options, 'Import');
+    $file_system->unlink($name);
+  }
+
+  /**
+   * Returns a .po file with a simple plural formula.
+   */
+  public function getPoFileWithSimplePlural() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "@count hour"
+msgid_plural "@count hours"
+msgstr[0] "@count heure"
+msgstr[1] "@count heures"
+
+msgid "@count second"
+msgid_plural "@count seconds"
+msgstr[0] "@count seconde"
+msgstr[1] "@count secondes"
+
+msgid "Monday"
+msgstr "lundi"
+EOF;
+  }
+
+  /**
+   * Returns a .po file with a complex plural formula.
+   */
+  public function getPoFileWithComplexPlural() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;\\n"
+
+msgid "@count hour"
+msgid_plural "@count hours"
+msgstr[0] "@count sat"
+msgstr[1] "@count sata"
+msgstr[2] "@count sati"
+
+msgid "Monday"
+msgstr "Ponedjeljak"
+EOF;
+  }
+
+  /**
+   * Returns a .po file with a missing plural formula.
+   */
+  public function getPoFileWithMissingPlural() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+
+msgid "Monday"
+msgstr "lundi"
+EOF;
+  }
+
+  /**
+   * Returns a .po file with a broken plural formula.
+   */
+  public function getPoFileWithBrokenPlural() {
+    return <<< EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: broken, will not parse\\n"
+
+msgid "Monday"
+msgstr "Ponedjeljak"
+EOF;
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleTranslatedSchemaDefinitionTest.php

@@ -0,0 +1,105 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Url;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\RequirementsPageTrait;
+
+/**
+ * Adds and configures languages to check field schema definition.
+ *
+ * @group locale
+ */
+class LocaleTranslatedSchemaDefinitionTest extends BrowserTestBase {
+
+  use RequirementsPageTrait;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'locale', 'node'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    ConfigurableLanguage::createFromLangcode('fr')->save();
+    $this->config('system.site')->set('default_langcode', 'fr')->save();
+
+    // Clear all caches so that the base field definition, its cache in the
+    // entity field manager, the t() cache, etc. are all cleared.
+    $this->resetAll();
+  }
+
+  /**
+   * Tests that translated field descriptions do not affect the update system.
+   */
+  public function testTranslatedSchemaDefinition(): void {
+    /** @var \Drupal\locale\StringDatabaseStorage $stringStorage */
+    $stringStorage = \Drupal::service('locale.storage');
+
+    $source = $stringStorage->createString([
+      'source' => 'Revision ID',
+    ])->save();
+
+    $stringStorage->createTranslation([
+      'lid' => $source->lid,
+      'language' => 'fr',
+      'translation' => 'Translated Revision ID',
+    ])->save();
+
+    // Ensure that the field is translated when access through the API.
+    $this->assertEquals('Translated Revision ID', \Drupal::service('entity_field.manager')->getBaseFieldDefinitions('node')['vid']->getLabel());
+
+    // Assert there are no updates.
+    $this->assertFalse(\Drupal::service('entity.definition_update_manager')->needsUpdates());
+  }
+
+  /**
+   * Tests that translations do not affect the update system.
+   */
+  public function testTranslatedUpdate(): void {
+    // Visit the update page to collect any strings that may be translatable.
+    $user = $this->drupalCreateUser(['administer software updates']);
+    $this->drupalLogin($user);
+    $update_url = Url::fromRoute('system.db_update')->setAbsolute()->toString();
+
+    // Collect strings from the PHP warning page, if applicable, as well as the
+    // main update page.
+    $this->drupalGet($update_url, ['external' => TRUE]);
+    $this->updateRequirementsProblem();
+
+    /** @var \Drupal\locale\StringDatabaseStorage $stringStorage */
+    $stringStorage = \Drupal::service('locale.storage');
+    $sources = $stringStorage->getStrings();
+
+    // Translate all source strings found.
+    foreach ($sources as $source) {
+      $stringStorage->createTranslation([
+        'lid' => $source->lid,
+        'language' => 'fr',
+        'translation' => $this->randomMachineName(100),
+      ])->save();
+    }
+
+    // Ensure that there are no updates just due to translations. Check for
+    // markup and a link instead of specific text because text may be
+    // translated.
+    $this->drupalGet($update_url . '/selection', ['external' => TRUE]);
+    $this->assertSession()->statusMessageExists('status');
+    $this->assertSession()->linkByHrefNotExists('fr/update.php/run', 'No link to run updates.');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleTranslationChangeProjectVersionTest.php

@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\StreamWrapper\PublicStream;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests how translations are handled when a project gets updated.
+ *
+ * @group locale
+ */
+class LocaleTranslationChangeProjectVersionTest extends LocaleUpdateBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    \Drupal::moduleHandler()->loadInclude('locale', 'inc', 'locale.batch');
+    ConfigurableLanguage::createFromLangcode('de')->save();
+
+    \Drupal::state()->set('locale.test_projects_alter', TRUE);
+    \Drupal::state()->set('locale.remove_core_project', TRUE);
+
+    // Setup the environment.
+    $config = $this->config('locale.settings');
+    $public_path = PublicStream::basePath();
+    $this->setTranslationsDirectory($public_path . '/local');
+    $config
+      ->set('translation.default_filename', '%project-%version.%language._po')
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+
+    // This test uses .po files for the old translation file instead of the ._po
+    // files because locale_translate_get_interface_translation_files() (used to
+    // delete old translation files) only works with .po files.
+    // The new translation file uses _po.
+    // Old version: 8.x-1.0; New version: 8.x-1.1.
+    $this->makePoFile('remote/all/contrib_module_one', 'contrib_module_one-8.x-1.0.de.po', $this->timestampOld, []);
+    $this->makePoFile('remote/all/contrib_module_one', 'contrib_module_one-8.x-1.1.de._po', $this->timestampNew, []);
+    $this->makePoFile('local', 'contrib_module_one-8.x-1.0.de.po', $this->timestampOld, []);
+
+    // Initialize the projects status and change the project version to the old
+    // version. This makes the code update the module translation to the new
+    // version when the (batch) update script is triggered.
+    $status = locale_translation_get_status();
+    $status['contrib_module_one']['de']->version = '8.x-1.0';
+    \Drupal::keyValue('locale.translation_status')->setMultiple($status);
+  }
+
+  /**
+   * Tests update translations when project version changes.
+   */
+  public function testUpdateImportSourceRemote(): void {
+
+    // Verify that the project status has the old version.
+    $status = locale_translation_get_status(['contrib_module_one']);
+    $this->assertEquals('8.x-1.0', $status['contrib_module_one']['de']->version);
+
+    // Verify that the old translation file exists and the new does not exist.
+    $this->assertFileExists('translations://contrib_module_one-8.x-1.0.de.po');
+    $this->assertFileDoesNotExist('translations://contrib_module_one-8.x-1.1.de._po');
+
+    // Run batch tasks.
+    $context = [];
+    locale_translation_batch_version_check('contrib_module_one', 'de', $context);
+    locale_translation_batch_status_check('contrib_module_one', 'de', [], $context);
+    locale_translation_batch_fetch_download('contrib_module_one', 'de', $context);
+
+    // Verify that the project status has the new version.
+    $status = locale_translation_get_status(['contrib_module_one']);
+    $this->assertEquals('8.x-1.1', $status['contrib_module_one']['de']->version);
+
+    // Verify that the old translation file was removed and the new was
+    // downloaded.
+    $this->assertFileDoesNotExist('translations://contrib_module_one-8.x-1.0.de.po');
+    $this->assertFileExists('translations://contrib_module_one-8.x-1.1.de._po');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleTranslationDownloadTest.php

@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\StreamWrapper\PublicStream;
+use Drupal\language\Entity\ConfigurableLanguage;
+use org\bovigo\vfs\vfsStream;
+
+/**
+ * Tests locale translation download.
+ *
+ * @group locale
+ */
+class LocaleTranslationDownloadTest extends LocaleUpdateBase {
+
+  /**
+   * The virtual file stream for storing translations.
+   *
+   * @var \org\bovigo\vfs\vfsStreamDirectory
+   */
+  protected $translationsStream;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $moduleHandler = $this->container->get('module_handler');
+    $moduleHandler->loadInclude('locale', 'inc', 'locale.batch');
+    ConfigurableLanguage::createFromLangcode('de')->save();
+
+    // Let the translations:// stream wrapper point to a virtual file system to
+    // make it independent from the test environment.
+    $this->translationsStream = vfsStream::setup('translations');
+    \Drupal::configFactory()->getEditable('locale.settings')
+      ->set('translation.path', $this->translationsStream->url())
+      ->save();
+  }
+
+  /**
+   * Tests translation download from remote sources.
+   */
+  public function testUpdateImportSourceRemote(): void {
+
+    // Provide remote and 'previously' downloaded translation file.
+    $this->setTranslationFiles();
+    vfsStream::create([
+      'contrib_module_one-8.x-1.1.de._po' => '__old_content__',
+    ], $this->translationsStream);
+
+    $url = \Drupal::service('url_generator')->generateFromRoute('<front>', [], ['absolute' => TRUE]);
+    $uri = $url . PublicStream::basePath() . '/remote/all/contrib_module_one/contrib_module_one-8.x-1.1.de._po';
+    $source_file = (object) [
+      'uri' => $uri,
+    ];
+
+    $result = locale_translation_download_source($source_file, 'translations://');
+
+    $this->assertEquals('translations://contrib_module_one-8.x-1.1.de._po', $result->uri);
+    $this->assertFileDoesNotExist('translations://contrib_module_one-8.x-1.1.de_0._po');
+    $this->assertFileExists('translations://contrib_module_one-8.x-1.1.de._po');
+    $this->assertStringNotContainsString('__old_content__', file_get_contents('translations://contrib_module_one-8.x-1.1.de._po'));
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleTranslationUiTest.php

@@ -0,0 +1,637 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Site\Settings;
+use Drupal\Core\Url;
+use Drupal\Core\Database\Database;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Core\Language\LanguageInterface;
+
+/**
+ * Tests the validation of translation strings and search results.
+ *
+ * @group locale
+ * @group #slow
+ */
+class LocaleTranslationUiTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Enable interface translation to English.
+   */
+  public function testEnglishTranslation(): void {
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($admin_user);
+
+    $this->drupalGet('admin/config/regional/language/edit/en');
+    $this->submitForm(['locale_translate_english' => TRUE], 'Save language');
+    $this->assertSession()->linkByHrefExists('/admin/config/regional/translate?langcode=en', 0, 'Enabled interface translation to English.');
+  }
+
+  /**
+   * Adds a language and tests string translation by users with the appropriate permissions.
+   */
+  public function testStringTranslation(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    // User to translate and delete string.
+    $translate_user = $this->drupalCreateUser([
+      'translate interface',
+      'access administration pages',
+    ]);
+    // Code for the language.
+    $langcode = 'xx';
+    // The English name for the language. This will be translated.
+    $name = 'Foo';
+    // This will be the translation of $name.
+    $translation = $this->randomMachineName(16);
+    $translation_to_en = $this->randomMachineName(16);
+
+    // Add custom language.
+    $this->drupalLogin($admin_user);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    // Add string.
+    t($name, [], ['langcode' => $langcode])->render();
+    // Reset locale cache.
+    $this->container->get('string_translation')->reset();
+    $this->assertSession()->responseContains('"edit-languages-' . $langcode . '-weight"');
+    // Ensure that test language was added.
+    $this->assertSession()->pageTextContains($name);
+    $this->drupalLogout();
+
+    // Add a whitespace at the end of string to ensure it is found.
+    $name_ws = $name . " ";
+
+    // Search for the name and translate it.
+    $this->drupalLogin($translate_user);
+    $search = [
+      'string' => $name_ws,
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    // Check that search finds the string as untranslated.
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($name);
+
+    // No t() here, it's surely not translated yet.
+    $this->assertSession()->pageTextContains($name);
+    // Verify that there is no way to translate the string to English.
+    $this->assertSession()->optionNotExists('edit-langcode', 'en');
+    $this->drupalLogout();
+    $this->drupalLogin($admin_user);
+    $this->drupalGet('admin/config/regional/language/edit/en');
+    $this->submitForm(['locale_translate_english' => TRUE], 'Save language');
+    $this->drupalLogout();
+    $this->drupalLogin($translate_user);
+    // Check that search finds the string as untranslated.
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($name);
+
+    // Assume this is the only result, given the random name.
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $translation,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+    $this->assertSession()->pageTextContains('The strings have been saved.');
+    $url_bits = explode('?', $this->getUrl());
+    $this->assertEquals(Url::fromRoute('locale.translate_page', [], ['absolute' => TRUE])->toString(), $url_bits[0], 'Correct page redirection.');
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($translation);
+
+    $search = [
+      'string' => $name,
+      'langcode' => 'en',
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $translation_to_en,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+    $search = [
+      'string' => $name,
+      'langcode' => 'en',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($translation_to_en);
+
+    $this->assertNotEquals($translation, $name);
+    $this->assertEquals($translation, t($name, [], ['langcode' => $langcode]), 't() works for non-English.');
+    // Refresh the locale() cache to get fresh data from t() below. We are in
+    // the same HTTP request and therefore t() is not refreshed by saving the
+    // translation above.
+    $this->container->get('string_translation')->reset();
+    // Now we should get the proper fresh translation from t().
+    $this->assertNotEquals($translation_to_en, $name);
+    $this->assertEquals($translation_to_en, t($name, [], ['langcode' => 'en']), 't() works for English.');
+    $this->assertTrue(t($name, [], ['langcode' => LanguageInterface::LANGCODE_SYSTEM]) == $name, 't() works for LanguageInterface::LANGCODE_SYSTEM.');
+
+    $search = [
+      'string' => $name,
+      'langcode' => 'en',
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Test invalidation of 'rendered' cache tag after string translation.
+    $this->drupalLogout();
+    $this->drupalGet('xx/user/login');
+    $this->assertSession()->pageTextContains('Password');
+
+    $this->drupalLogin($translate_user);
+    $search = [
+      'string' => 'Password',
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => 'Llamas are larger than frogs.',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    $this->drupalLogout();
+    $this->drupalGet('xx/user/login');
+    $this->assertSession()->pageTextContains('Llamas are larger than frogs.');
+
+    // Delete the language.
+    $this->drupalLogin($admin_user);
+    $path = 'admin/config/regional/language/delete/' . $langcode;
+    // This a confirm form, we do not need any fields changed.
+    $this->drupalGet($path);
+    $this->submitForm([], 'Delete');
+    $this->assertSession()->pageTextContains("The {$name} ({$langcode}) language has been removed.");
+    // Reload to remove $name.
+    $this->drupalGet($path);
+    // Verify that language is no longer found.
+    $this->assertSession()->statusCodeEquals(404);
+    $this->drupalLogout();
+
+    // Delete the string.
+    $this->drupalLogin($translate_user);
+    $search = [
+      'string' => $name,
+      'langcode' => 'en',
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    // Assume this is the only result, given the random name.
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => '',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+    $this->assertSession()->responseContains($name);
+    $this->drupalLogin($translate_user);
+    $search = [
+      'string' => $name,
+      'langcode' => 'en',
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+  }
+
+  /**
+   * Tests the rebuilding of JavaScript translation files on deletion.
+   */
+  public function testJavaScriptTranslation(): void {
+    $user = $this->drupalCreateUser([
+      'translate interface',
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($user);
+    $config = $this->config('locale.settings');
+
+    $langcode = 'xx';
+    // The English name for the language. This will be translated.
+    $name = $this->randomMachineName(16);
+
+    // Add custom language.
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    $this->container->get('language_manager')->reset();
+
+    // Build the JavaScript translation file.
+
+    // Retrieve the source string of the first string available in the
+    // {locales_source} table and translate it.
+    $query = Database::getConnection()->select('locales_source', 's');
+    $query->addJoin('INNER', 'locales_location', 'l', '[s].[lid] = [l].[lid]');
+    $source = $query->fields('s', ['source'])
+      ->condition('l.type', 'javascript')
+      ->range(0, 1)
+      ->execute()
+      ->fetchField();
+
+    $search = [
+      'string' => $source,
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $this->randomMachineName(),
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Trigger JavaScript translation parsing and building.
+    _locale_rebuild_js($langcode);
+
+    $locale_javascripts = \Drupal::state()->get('locale.translation.javascript', []);
+    $js_file = 'public://' . $config->get('javascript.directory') . '/' . $langcode . '_' . $locale_javascripts[$langcode] . '.js';
+    $this->assertFileExists($js_file);
+
+    // Test JavaScript translation rebuilding.
+    \Drupal::service('file_system')->delete($js_file);
+    $this->assertFileDoesNotExist($js_file);
+    _locale_rebuild_js($langcode);
+    $this->assertFileExists($js_file);
+
+    // Test if JavaScript translation contains a custom string override.
+    $string_override = $this->randomMachineName();
+    $settings = Settings::getAll();
+    $settings['locale_custom_strings_' . $langcode] = ['' => [$string_override => $string_override]];
+    // Recreate the settings static.
+    new Settings($settings);
+    _locale_rebuild_js($langcode);
+    $locale_javascripts = \Drupal::state()->get('locale.translation.javascript', []);
+    $js_file = 'public://' . $config->get('javascript.directory') . '/' . $langcode . '_' . $locale_javascripts[$langcode] . '.js';
+    $content = file_get_contents($js_file);
+    $this->assertStringContainsString('"' . $string_override . '":"' . $string_override . '"', $content);
+  }
+
+  /**
+   * Tests the validation of the translation input.
+   */
+  public function testStringValidation(): void {
+    // User to add language and strings.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+    $langcode = 'xx';
+    // The English name for the language. This will be translated.
+    $name = $this->randomMachineName(16);
+
+    // These will be the invalid translations of $name.
+    $key = $this->randomMachineName(16);
+    $bad_translations[$key] = "<script>alert('xss');</script>" . $key;
+    $key = $this->randomMachineName(16);
+    $bad_translations[$key] = '<img SRC="javascript:alert(\'xss\');">' . $key;
+    $key = $this->randomMachineName(16);
+    $bad_translations[$key] = '<<SCRIPT>alert("xss");//<</SCRIPT>' . $key;
+    $key = $this->randomMachineName(16);
+    $bad_translations[$key] = "<BODY ONLOAD=alert('xss')>" . $key;
+
+    // Add custom language.
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+    // Add string.
+    t($name, [], ['langcode' => $langcode])->render();
+    // Reset locale cache.
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    // Find the edit path.
+
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    foreach ($bad_translations as $translation) {
+      $edit = [
+        $lid => $translation,
+      ];
+      $this->drupalGet('admin/config/regional/translate');
+      $this->submitForm($edit, 'Save translations');
+      // Check for a form error on the textarea, which means the string was
+      // rejected as unsafe.
+      $this->assertSession()->elementAttributeContains('xpath', '//form[@id="locale-translate-edit-form"]//textarea', 'class', 'error');
+      $this->assertSession()->pageTextNotContains('The string has been saved.');
+    }
+  }
+
+  /**
+   * Tests translation search form.
+   */
+  public function testStringSearch(): void {
+    // User to add and remove language.
+    $admin_user = $this->drupalCreateUser([
+      'administer languages',
+      'access administration pages',
+    ]);
+    // User to translate and delete string.
+    $translate_user = $this->drupalCreateUser([
+      'translate interface',
+      'access administration pages',
+    ]);
+
+    // Code for the language.
+    $langcode = 'xx';
+    // The English name for the language. This will be translated.
+    $name = $this->randomMachineName(16);
+    // This will be the translation of $name.
+    $translation = $this->randomMachineName(16);
+
+    // Add custom language.
+    $this->drupalLogin($admin_user);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => 'yy',
+      'label' => $this->randomMachineName(16),
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Add string.
+    t($name, [], ['langcode' => $langcode])->render();
+    // Reset locale cache.
+    $this->container->get('string_translation')->reset();
+    $this->drupalLogout();
+
+    // Search for the name.
+    $this->drupalLogin($translate_user);
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    // pageTextContains() seems to remove the input field where $name always
+    // could be found, so this is not a false assert. See how
+    // pageTextNotContains succeeds later.
+    $this->assertSession()->pageTextContains($name);
+
+    // Ensure untranslated string doesn't appear if searching on 'only
+    // translated strings'.
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Ensure untranslated string appears if searching on 'only untranslated
+    // strings'.
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+
+    // Add translation.
+    // Assume this is the only result, given the random name.
+    // We save the lid from the path.
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $translation,
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Ensure translated string does appear if searching on 'only
+    // translated strings'.
+    $search = [
+      'string' => $translation,
+      'langcode' => $langcode,
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+
+    // Ensure translated source string doesn't appear if searching on 'only
+    // untranslated strings'.
+    $search = [
+      'string' => $name,
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Ensure translated string doesn't appear if searching on 'only
+    // untranslated strings'.
+    $search = [
+      'string' => $translation,
+      'langcode' => $langcode,
+      'translation' => 'untranslated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Ensure translated string does appear if searching on the custom language.
+    $search = [
+      'string' => $translation,
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+
+    // Ensure translated string doesn't appear if searching in System (English).
+    $search = [
+      'string' => $translation,
+      'langcode' => 'yy',
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+
+    // Search for a string that isn't in the system.
+    $unavailable_string = $this->randomMachineName(16);
+    $search = [
+      'string' => $unavailable_string,
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('No strings available.');
+  }
+
+  /**
+   * Tests that only changed strings are saved customized when edited.
+   */
+  public function testUICustomizedStrings(): void {
+    $user = $this->drupalCreateUser([
+      'translate interface',
+      'administer languages',
+      'access administration pages',
+    ]);
+    $this->drupalLogin($user);
+    ConfigurableLanguage::createFromLangcode('de')->save();
+
+    // Create test source string.
+    $string = $this->container->get('locale.storage')->createString([
+      'source' => $this->randomMachineName(100),
+      'context' => $this->randomMachineName(20),
+    ])->save();
+
+    // Create translation for new string and save it as non-customized.
+    $translation = $this->container->get('locale.storage')->createTranslation([
+      'lid' => $string->lid,
+      'language' => 'de',
+      'translation' => $this->randomMachineName(100),
+      'customized' => 0,
+    ])->save();
+
+    // Reset locale cache.
+    $this->container->get('string_translation')->reset();
+
+    // Ensure non-customized translation string does appear if searching
+    // non-customized translation.
+    $search = [
+      'string' => $string->getString(),
+      'langcode' => 'de',
+      'translation' => 'translated',
+      'customized' => '0',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+
+    $this->assertSession()->pageTextContains($translation->getString());
+
+    // Submit the translations without changing the translation.
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $translation->getString(),
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Ensure unchanged translation string does appear if searching
+    // non-customized translation.
+    $search = [
+      'string' => $string->getString(),
+      'langcode' => 'de',
+      'translation' => 'translated',
+      'customized' => '0',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($string->getString());
+
+    // Submit the translations with a new translation.
+    $textarea = $this->assertSession()->elementExists('xpath', '//textarea');
+    $lid = $textarea->getAttribute('name');
+    $edit = [
+      $lid => $this->randomMachineName(100),
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($edit, 'Save translations');
+
+    // Ensure changed translation string does appear if searching customized
+    // translation.
+    $search = [
+      'string' => $string->getString(),
+      'langcode' => 'de',
+      'translation' => 'translated',
+      'customized' => '1',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains($string->getString());
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleUpdateBase.php

@@ -0,0 +1,322 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\StreamWrapper\PublicStream;
+use Drupal\file\Entity\File;
+use Drupal\Tests\BrowserTestBase;
+
+// cspell:ignore februar januar juni marz
+
+/**
+ * Base class for testing updates to string translations.
+ */
+abstract class LocaleUpdateBase extends BrowserTestBase {
+
+  /**
+   * Timestamp for an old translation.
+   *
+   * @var int
+   */
+  protected $timestampOld;
+
+  /**
+   * Timestamp for a medium aged translation.
+   *
+   * @var int
+   */
+  protected $timestampMedium;
+
+  /**
+   * Timestamp for a new translation.
+   *
+   * @var int
+   */
+  protected $timestampNew;
+
+  /**
+   * Timestamp for current time.
+   *
+   * @var int
+   */
+  protected $timestampNow;
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'locale_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Setup timestamps to identify old and new translation sources.
+    $this->timestampOld = \Drupal::time()->getRequestTime() - 300;
+    $this->timestampMedium = \Drupal::time()->getRequestTime() - 200;
+    $this->timestampNew = \Drupal::time()->getRequestTime() - 100;
+    $this->timestampNow = \Drupal::time()->getRequestTime();
+
+    // Enable import of translations. By default this is disabled for automated
+    // tests.
+    $this->config('locale.settings')
+      ->set('translation.import_enabled', TRUE)
+      ->set('translation.use_source', LOCALE_TRANSLATION_USE_SOURCE_LOCAL)
+      ->save();
+  }
+
+  /**
+   * Sets the value of the default translations directory.
+   *
+   * @param string $path
+   *   Path of the translations directory relative to the drupal installation
+   *   directory.
+   */
+  protected function setTranslationsDirectory($path) {
+    \Drupal::service('file_system')->prepareDirectory($path, FileSystemInterface::CREATE_DIRECTORY);
+    $this->config('locale.settings')->set('translation.path', $path)->save();
+  }
+
+  /**
+   * Adds a language.
+   *
+   * @param string $langcode
+   *   The language code of the language to add.
+   */
+  protected function addLanguage($langcode) {
+    $edit = ['predefined_langcode' => $langcode];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+    $this->container->get('language_manager')->reset();
+    $this->assertNotEmpty(\Drupal::languageManager()->getLanguage($langcode), "Language $langcode added.");
+  }
+
+  /**
+   * Creates a translation file and tests its timestamp.
+   *
+   * @param string $path
+   *   Path of the file relative to the public file path.
+   * @param string $filename
+   *   Name of the file to create.
+   * @param int $timestamp
+   *   (optional) Timestamp to set the file to. Defaults to current time.
+   * @param array $translations
+   *   (optional) Array of source/target value translation strings. Only
+   *   singular strings are supported, no plurals. No double quotes are allowed
+   *   in source and translations strings.
+   */
+  protected function makePoFile($path, $filename, $timestamp = NULL, array $translations = []) {
+    $timestamp = $timestamp ? $timestamp : \Drupal::time()->getRequestTime();
+    $path = 'public://' . $path;
+    $text = '';
+    $po_header = <<<EOF
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+EOF;
+
+    // Convert array of translations to Gettext source and translation strings.
+    if ($translations) {
+      foreach ($translations as $source => $target) {
+        $text .= 'msgid "' . $source . '"' . "\n";
+        $text .= 'msgstr "' . $target . '"' . "\n";
+      }
+    }
+
+    \Drupal::service('file_system')->prepareDirectory($path, FileSystemInterface::CREATE_DIRECTORY);
+    $fileUri = $path . '/' . $filename;
+    $file = File::create([
+      'uid' => 1,
+      'filename' => $filename,
+      'uri' => $fileUri,
+      'filemime' => 'text/x-gettext-translation',
+      'timestamp' => $timestamp,
+    ]);
+    $file->setPermanent();
+    file_put_contents($file->getFileUri(), $po_header . $text);
+    touch(\Drupal::service('file_system')->realpath($file->getFileUri()), $timestamp);
+    $file->save();
+
+    $this->assertTrue(file_exists($fileUri));
+    $this->assertEquals($timestamp, filemtime($fileUri));
+  }
+
+  /**
+   * Setup the environment containing local and remote translation files.
+   *
+   * Update tests require a simulated environment for local and remote files.
+   * Normally remote files are located at a remote server (e.g. ftp.drupal.org).
+   * For testing we can not rely on this. A directory in the file system of the
+   * test site is designated for remote files and is addressed using an absolute
+   * URL. Because Drupal does not allow files with a po extension to be accessed
+   * (denied in .htaccess) the translation files get a _po extension. Another
+   * directory is designated for local translation files.
+   *
+   * The environment is set up with the following files. File creation times are
+   * set to create different variations in test conditions.
+   *   contrib_module_one
+   *    - remote file: timestamp new
+   *    - local file:  timestamp old
+   *   contrib_module_two
+   *    - remote file: timestamp old
+   *    - local file:  timestamp new
+   *   contrib_module_three
+   *    - remote file: timestamp old
+   *    - local file:  timestamp old
+   *   custom_module_one
+   *    - local file:  timestamp new
+   * Time stamp of current translation set by setCurrentTranslations() is always
+   * timestamp medium. This makes it easy to predict which translation will be
+   * imported.
+   */
+  protected function setTranslationFiles() {
+    $config = $this->config('locale.settings');
+
+    // A flag is set to let the locale_test module replace the project data with
+    // a set of test projects which match the below project files.
+    \Drupal::state()->set('locale.test_projects_alter', TRUE);
+    \Drupal::state()->set('locale.remove_core_project', FALSE);
+
+    // Setup the environment.
+    $public_path = PublicStream::basePath();
+    $this->setTranslationsDirectory($public_path . '/local');
+    $config->set('translation.default_filename', '%project-%version.%language._po')->save();
+
+    // Setting up sets of translations for the translation files.
+    $translations_one = ['January' => 'Januar_1', 'February' => 'Februar_1', 'March' => 'Marz_1'];
+    $translations_two = ['February' => 'Februar_2', 'March' => 'Marz_2', 'April' => 'April_2'];
+    $translations_three = ['April' => 'April_3', 'May' => 'Mai_3', 'June' => 'Juni_3'];
+
+    // Add a number of files to the local file system to serve as remote
+    // translation server and match the project definitions set in
+    // locale_test_locale_translation_projects_alter().
+    $this->makePoFile('remote/all/contrib_module_one', 'contrib_module_one-8.x-1.1.de._po', $this->timestampNew, $translations_one);
+    $this->makePoFile('remote/all/contrib_module_two', 'contrib_module_two-8.x-2.0-beta4.de._po', $this->timestampOld, $translations_two);
+    $this->makePoFile('remote/all/contrib_module_three', 'contrib_module_three-8.x-1.0.de._po', $this->timestampOld, $translations_three);
+
+    // Add a number of files to the local file system to serve as local
+    // translation files and match the project definitions set in
+    // locale_test_locale_translation_projects_alter().
+    $this->makePoFile('local', 'contrib_module_one-8.x-1.1.de._po', $this->timestampOld, $translations_one);
+    $this->makePoFile('local', 'contrib_module_two-8.x-2.0-beta4.de._po', $this->timestampNew, $translations_two);
+    $this->makePoFile('local', 'contrib_module_three-8.x-1.0.de._po', $this->timestampOld, $translations_three);
+    $this->makePoFile('local', 'custom_module_one.de.po', $this->timestampNew);
+  }
+
+  /**
+   * Sets up existing translations and their statuses in the database.
+   */
+  protected function setCurrentTranslations() {
+    // Add non customized translations to the database.
+    $langcode = 'de';
+    $context = '';
+    $non_customized_translations = [
+      'March' => 'Marz',
+      'June' => 'Juni',
+    ];
+    foreach ($non_customized_translations as $source => $translation) {
+      $string = $this->container->get('locale.storage')->createString([
+        'source' => $source,
+        'context' => $context,
+      ])
+        ->save();
+      $this->container->get('locale.storage')->createTranslation([
+        'lid' => $string->getId(),
+        'language' => $langcode,
+        'translation' => $translation,
+        'customized' => LOCALE_NOT_CUSTOMIZED,
+      ])->save();
+    }
+
+    // Add customized translations to the database.
+    $customized_translations = [
+      'January' => 'Januar_customized',
+      'February' => 'Februar_customized',
+      'May' => 'Mai_customized',
+    ];
+    foreach ($customized_translations as $source => $translation) {
+      $string = $this->container->get('locale.storage')->createString([
+        'source' => $source,
+        'context' => $context,
+      ])
+        ->save();
+      $this->container->get('locale.storage')->createTranslation([
+        'lid' => $string->getId(),
+        'language' => $langcode,
+        'translation' => $translation,
+        'customized' => LOCALE_CUSTOMIZED,
+      ])->save();
+    }
+
+    // Add a state of current translations in locale_files.
+    $default = [
+      'langcode' => $langcode,
+      'uri' => '',
+      'timestamp' => $this->timestampMedium,
+      'last_checked' => $this->timestampMedium,
+    ];
+    $data[] = [
+      'project' => 'contrib_module_one',
+      'filename' => 'contrib_module_one-8.x-1.1.de._po',
+      'version' => '8.x-1.1',
+    ];
+    $data[] = [
+      'project' => 'contrib_module_two',
+      'filename' => 'contrib_module_two-8.x-2.0-beta4.de._po',
+      'version' => '8.x-2.0-beta4',
+    ];
+    $data[] = [
+      'project' => 'contrib_module_three',
+      'filename' => 'contrib_module_three-8.x-1.0.de._po',
+      'version' => '8.x-1.0',
+    ];
+    $data[] = [
+      'project' => 'custom_module_one',
+      'filename' => 'custom_module_one.de.po',
+      'version' => '',
+    ];
+    $connection = Database::getConnection();
+    foreach ($data as $file) {
+      $file = array_merge($default, $file);
+      $connection->insert('locale_file')->fields($file)->execute();
+    }
+  }
+
+  /**
+   * Checks the translation of a string.
+   *
+   * @param string $source
+   *   Translation source string.
+   * @param string $translation
+   *   Translation to check. Use empty string to check for a non-existent
+   *   translation.
+   * @param string $langcode
+   *   Language code of the language to translate to.
+   * @param string $message
+   *   (optional) A message to display with the assertion.
+   */
+  protected function assertTranslation($source, $translation, $langcode, $message = '') {
+    $query = Database::getConnection()->select('locales_target', 'lt');
+    $query->innerJoin('locales_source', 'ls', '[ls].[lid] = [lt].[lid]');
+    $db_translation = $query->fields('lt', ['translation'])
+      ->condition('ls.source', $source)
+      ->condition('lt.language', $langcode)
+      ->execute()
+      ->fetchField();
+    $db_translation = $db_translation == FALSE ? '' : $db_translation;
+    $this->assertEquals($translation, $db_translation, $message ?: "Correct translation of $source ($langcode)");
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleUpdateCronTest.php

@@ -0,0 +1,137 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\Tests\Traits\Core\CronRunTrait;
+
+/**
+ * Tests for using cron to update project interface translations.
+ *
+ * @group locale
+ */
+class LocaleUpdateCronTest extends LocaleUpdateBase {
+
+  use CronRunTrait;
+
+  protected $batchOutput = [];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+    $this->addLanguage('de');
+  }
+
+  /**
+   * Tests interface translation update using cron.
+   */
+  public function testUpdateCron(): void {
+    // Set a flag to let the locale_test module replace the project data with a
+    // set of test projects.
+    \Drupal::state()->set('locale.test_projects_alter', TRUE);
+
+    // Setup local and remote translations files.
+    $this->setTranslationFiles();
+    $this->config('locale.settings')->set('translation.default_filename', '%project-%version.%language._po')->save();
+
+    // Update translations using batch to ensure a clean test starting point.
+    $this->drupalGet('admin/reports/translations/check');
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Store translation status for comparison.
+    $initial_history = locale_translation_get_file_history();
+
+    // Prepare for test: Simulate new translations being available.
+    // Change the last updated timestamp of a translation file.
+    $contrib_module_two_uri = 'public://local/contrib_module_two-8.x-2.0-beta4.de._po';
+    touch(\Drupal::service('file_system')->realpath($contrib_module_two_uri), \Drupal::time()->getRequestTime());
+
+    // Prepare for test: Simulate that the file has not been checked for a long
+    // time. Set the last_check timestamp to zero.
+    $query = Database::getConnection()->update('locale_file');
+    $query->fields(['last_checked' => 0]);
+    $query->condition('project', 'contrib_module_two');
+    $query->condition('langcode', 'de');
+    $query->execute();
+
+    // Test: Disable cron update and verify that no tasks are added to the
+    // queue.
+    $edit = [
+      'update_interval_days' => 0,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Execute locale cron tasks to add tasks to the queue.
+    locale_cron();
+
+    // Check whether no tasks are added to the queue.
+    $queue = \Drupal::queue('locale_translation', TRUE);
+    $this->assertEquals(0, $queue->numberOfItems(), 'Queue is empty');
+
+    // Test: Enable cron update and check if update tasks are added to the
+    // queue.
+    // Set cron update to Weekly.
+    $edit = [
+      'update_interval_days' => 7,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Execute locale cron tasks to add tasks to the queue.
+    locale_cron();
+
+    // Check whether tasks are added to the queue.
+    // Expected tasks:
+    // - locale_translation_batch_version_check
+    // - locale_translation_batch_status_check
+    // - locale_translation_batch_status_finished.
+    $queue = \Drupal::queue('locale_translation', TRUE);
+    $this->assertEquals(3, $queue->numberOfItems(), 'Queue holds tasks for one project.');
+    $item = $queue->claimItem();
+    $queue->releaseItem($item);
+    $this->assertEquals('contrib_module_two', $item->data[1][0], 'Queue holds tasks for contrib module one.');
+
+    // Test: Run cron for a second time and check if tasks are not added to
+    // the queue twice.
+    locale_cron();
+
+    // Check whether no more tasks are added to the queue.
+    $queue = \Drupal::queue('locale_translation', TRUE);
+    $this->assertEquals(3, $queue->numberOfItems(), 'Queue holds tasks for one project.');
+
+    // Ensure last checked is updated to a greater time than the initial value.
+    sleep(1);
+    // Test: Execute cron and check if tasks are executed correctly.
+    // Run cron to process the tasks in the queue.
+    $this->cronRun();
+
+    drupal_static_reset('locale_translation_get_file_history');
+    $history = locale_translation_get_file_history();
+    $initial = $initial_history['contrib_module_two']['de'];
+    $current = $history['contrib_module_two']['de'];
+    // Verify that the translation of contrib_module_one is imported and
+    // updated.
+    $this->assertGreaterThan($initial->timestamp, $current->timestamp);
+    $this->assertGreaterThan($initial->last_checked, $current->last_checked);
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleUpdateDevelopmentReleaseTest.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Test for proper version fallback in case of a development release.
+ *
+ * @group language
+ */
+class LocaleUpdateDevelopmentReleaseTest extends BrowserTestBase {
+
+  protected static $modules = ['locale', 'locale_test_development_release'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    \Drupal::moduleHandler()->loadInclude('locale', 'inc', 'locale.compare');
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm(['predefined_langcode' => 'hu'], 'Add language');
+  }
+
+  public function testLocaleUpdateDevelopmentRelease(): void {
+    $projects = locale_translation_build_projects();
+    $this->assertEquals('8.0.x', $projects['drupal']->info['version'], 'The branch of the core dev release.');
+    $this->assertEquals('12.x-10.x', $projects['contrib']->info['version'], 'The branch of the contrib module dev release.');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleUpdateInterfaceTest.php

@@ -0,0 +1,129 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Url;
+
+/**
+ * Tests for the user interface of project interface translations.
+ *
+ * @group locale
+ */
+class LocaleUpdateInterfaceTest extends LocaleUpdateBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale_test_translate'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+  }
+
+  /**
+   * Tests the user interfaces of the interface translation update system.
+   *
+   * Testing the Available updates summary on the side wide status page and the
+   * Available translation updates page.
+   */
+  public function testInterface(): void {
+    // No language added.
+    // Check status page and Available translation updates page.
+    $this->drupalGet('admin/reports/status');
+    $this->assertSession()->pageTextNotContains('Translation update status');
+
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains("No translatable languages available. Add a language first.");
+    $this->assertSession()->linkByHrefExists(Url::fromRoute('entity.configurable_language.collection')->toString());
+
+    // Add German language.
+    $this->addLanguage('de');
+
+    // Override Drupal core translation status as 'up-to-date'.
+    $status = locale_translation_get_status();
+    $status['drupal']['de']->type = 'current';
+    \Drupal::keyValue('locale.translation_status')->set('drupal', $status['drupal']);
+
+    // One language added, all translations up to date.
+    $this->drupalGet('admin/reports/status');
+    $this->assertSession()->pageTextContains('Translation update status');
+    $this->assertSession()->pageTextContains('Up to date');
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('All translations up to date.');
+
+    // Set locale_test_translate module to have a local translation available.
+    $status = locale_translation_get_status();
+    $status['locale_test_translate']['de']->type = 'local';
+    \Drupal::keyValue('locale.translation_status')->set('locale_test_translate', $status['locale_test_translate']);
+
+    // Check if updates are available for German.
+    $this->drupalGet('admin/reports/status');
+    $this->assertSession()->pageTextContains('Translation update status');
+    $this->assertSession()->pageTextContains("Updates available for: German. See the Available translation updates page for more information.");
+    $this->assertSession()->linkByHrefExists(Url::fromRoute('locale.translate_status')->toString());
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('Updates for: Locale test translate');
+
+    // Set locale_test_translate module to have a dev release and no
+    // translation found.
+    $status = locale_translation_get_status();
+    $status['locale_test_translate']['de']->version = '1.3-dev';
+    $status['locale_test_translate']['de']->type = '';
+    \Drupal::keyValue('locale.translation_status')->set('locale_test_translate', $status['locale_test_translate']);
+
+    // Check if no updates were found.
+    $this->drupalGet('admin/reports/status');
+    $this->assertSession()->pageTextContains('Translation update status');
+    $this->assertSession()->pageTextContains("Missing translations for: German. See the Available translation updates page for more information.");
+    $this->assertSession()->linkByHrefExists(Url::fromRoute('locale.translate_status')->toString());
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('Missing translations for one project');
+    $this->assertSession()->pageTextContains('Locale test translate (1.3-dev). File not found at core/modules/locale/tests/test.de.po');
+
+    // Override Drupal core translation status as 'no translations found'.
+    $status = locale_translation_get_status();
+    $status['drupal']['de']->type = '';
+    $status['drupal']['de']->timestamp = 0;
+    $status['drupal']['de']->version = '8.1.1';
+    \Drupal::keyValue('locale.translation_status')->set('drupal', $status['drupal']);
+
+    // Check if Drupal core is not translated.
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('Missing translations for 2 projects');
+    $this->assertSession()->pageTextContains('Drupal core (8.1.1).');
+
+    // Override Drupal core translation status as 'translations available'.
+    $status = locale_translation_get_status();
+    $status['drupal']['de']->type = 'local';
+    $status['drupal']['de']->files['local']->timestamp = \Drupal::time()->getRequestTime();
+    $status['drupal']['de']->files['local']->info['version'] = '8.1.1';
+    \Drupal::keyValue('locale.translation_status')->set('drupal', $status['drupal']);
+
+    // Check if translations are available for Drupal core.
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('Updates for: Drupal core');
+    $this->assertSession()->pageTextContains('Drupal core (' . $this->container->get('date.formatter')->format(\Drupal::time()->getRequestTime(), 'html_date') . ')');
+    $this->assertSession()->buttonExists('Update translations');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocaleUpdateTest.php

@@ -0,0 +1,487 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\Core\Database\Database;
+use Drupal\Core\Language\LanguageInterface;
+
+// cspell:ignore extraday lundi
+
+/**
+ * Tests for updating the interface translations of projects.
+ *
+ * @group locale
+ * @group #slow
+ */
+class LocaleUpdateTest extends LocaleUpdateBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $module_handler = \Drupal::moduleHandler();
+    $module_handler->loadInclude('locale', 'inc', 'locale.compare');
+    $module_handler->loadInclude('locale', 'inc', 'locale.fetch');
+    $admin_user = $this->drupalCreateUser([
+      'administer modules',
+      'administer site configuration',
+      'administer languages',
+      'access administration pages',
+      'translate interface',
+    ]);
+    $this->drupalLogin($admin_user);
+    // We use German as test language. This language must match the translation
+    // file that come with the locale_test module (test.de.po) and can therefore
+    // not be chosen randomly.
+    $this->addLanguage('de');
+  }
+
+  /**
+   * Checks if local or remote translation sources are detected.
+   *
+   * The translation status process by default checks the status of the
+   * installed projects. For testing purpose a predefined set of modules with
+   * fixed file names and release versions is used. This custom project
+   * definition is applied using a hook_locale_translation_projects_alter
+   * implementation in the locale_test module.
+   *
+   * This test generates a set of local and remote translation files in their
+   * respective local and remote translation directory. The test checks whether
+   * the most recent files are selected in the different check scenarios: check
+   * for local files only, check for both local and remote files.
+   */
+  public function testUpdateCheckStatus(): void {
+    // Case when contributed modules are absent.
+    $this->drupalGet('admin/reports/translations');
+    $this->assertSession()->pageTextContains('Missing translations for one project');
+
+    $config = $this->config('locale.settings');
+    // Set a flag to let the locale_test module replace the project data with a
+    // set of test projects.
+    \Drupal::state()->set('locale.test_projects_alter', TRUE);
+
+    // Create local and remote translations files.
+    $this->setTranslationFiles();
+    $config->set('translation.default_filename', '%project-%version.%language._po')->save();
+
+    // Set the test conditions.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_LOCAL,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Get status of translation sources at local file system.
+    $this->drupalGet('admin/reports/translations/check');
+    $result = locale_translation_get_status();
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['contrib_module_one']['de']->type, 'Translation of contrib_module_one found');
+    $this->assertEquals($this->timestampOld, $result['contrib_module_one']['de']->timestamp, 'Translation timestamp found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['contrib_module_two']['de']->type, 'Translation of contrib_module_two found');
+    $this->assertEquals($this->timestampNew, $result['contrib_module_two']['de']->timestamp, 'Translation timestamp found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['locale_test']['de']->type, 'Translation of locale_test found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['custom_module_one']['de']->type, 'Translation of custom_module_one found');
+
+    // Set the test conditions.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_REMOTE_AND_LOCAL,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Get status of translation sources at both local and remote locations.
+    $this->drupalGet('admin/reports/translations/check');
+    $result = locale_translation_get_status();
+    $this->assertEquals(LOCALE_TRANSLATION_REMOTE, $result['contrib_module_one']['de']->type, 'Translation of contrib_module_one found');
+    $this->assertEquals($this->timestampNew, $result['contrib_module_one']['de']->timestamp, 'Translation timestamp found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['contrib_module_two']['de']->type, 'Translation of contrib_module_two found');
+    $this->assertEquals($this->timestampNew, $result['contrib_module_two']['de']->timestamp, 'Translation timestamp found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['contrib_module_three']['de']->type, 'Translation of contrib_module_three found');
+    $this->assertEquals($this->timestampOld, $result['contrib_module_three']['de']->timestamp, 'Translation timestamp found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['locale_test']['de']->type, 'Translation of locale_test found');
+    $this->assertEquals(LOCALE_TRANSLATION_LOCAL, $result['custom_module_one']['de']->type, 'Translation of custom_module_one found');
+  }
+
+  /**
+   * Tests translation import from remote sources.
+   *
+   * Test conditions:
+   *  - Source: remote and local files
+   *  - Import overwrite: all existing translations
+   */
+  public function testUpdateImportSourceRemote(): void {
+    $config = $this->config('locale.settings');
+
+    // Build the test environment.
+    $this->setTranslationFiles();
+    $this->setCurrentTranslations();
+    $config->set('translation.default_filename', '%project-%version.%language._po');
+
+    // Set the update conditions for this test.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_REMOTE_AND_LOCAL,
+      'overwrite' => LOCALE_TRANSLATION_OVERWRITE_ALL,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Get the translation status.
+    $this->drupalGet('admin/reports/translations/check');
+
+    // Check the status on the Available translation status page.
+    $this->assertSession()->responseContains('<label for="edit-langcodes-de" class="visually-hidden">Update German</label>');
+    $this->assertSession()->pageTextContains('Updates for: Contributed module one, Contributed module two, Custom module one, Locale test');
+    /** @var \Drupal\Core\Datetime\DateFormatterInterface $date_formatter */
+    $date_formatter = $this->container->get('date.formatter');
+    $this->assertSession()->pageTextContains('Contributed module one (' . $date_formatter->format($this->timestampNew, 'html_date') . ')');
+    $this->assertSession()->pageTextContains('Contributed module two (' . $date_formatter->format($this->timestampNew, 'html_date') . ')');
+
+    // Execute the translation update.
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check if the translation has been updated, using the status cache.
+    $status = locale_translation_get_status();
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_one']['de']->type, 'Translation of contrib_module_one found');
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_two']['de']->type, 'Translation of contrib_module_two found');
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_three']['de']->type, 'Translation of contrib_module_three found');
+
+    // Check the new translation status.
+    // The static cache needs to be flushed first to get the most recent data
+    // from the database. The function was called earlier during this test.
+    drupal_static_reset('locale_translation_get_file_history');
+    $history = locale_translation_get_file_history();
+    // Verify that the translation of contrib_module_one is imported and
+    // updated.
+    $this->assertGreaterThanOrEqual($this->timestampNow, $history['contrib_module_one']['de']->timestamp);
+    $this->assertGreaterThanOrEqual($this->timestampNow, $history['contrib_module_one']['de']->last_checked);
+    $this->assertEquals($this->timestampNew, $history['contrib_module_two']['de']->timestamp, 'Translation of contrib_module_two is imported');
+    // Verify that the translation of contrib_module_two is updated.
+    $this->assertGreaterThanOrEqual($this->timestampNow, $history['contrib_module_two']['de']->last_checked);
+    $this->assertEquals($this->timestampMedium, $history['contrib_module_three']['de']->timestamp, 'Translation of contrib_module_three is not imported');
+    $this->assertEquals($this->timestampMedium, $history['contrib_module_three']['de']->last_checked, 'Translation of contrib_module_three is not updated');
+
+    // Check whether existing translations have (not) been overwritten.
+    // cSpell:disable
+    $this->assertEquals('Januar_1', t('January', [], ['langcode' => 'de']), 'Translation of January');
+    $this->assertEquals('Februar_2', t('February', [], ['langcode' => 'de']), 'Translation of February');
+    $this->assertEquals('Marz_2', t('March', [], ['langcode' => 'de']), 'Translation of March');
+    $this->assertEquals('April_2', t('April', [], ['langcode' => 'de']), 'Translation of April');
+    $this->assertEquals('Mai_customized', t('May', [], ['langcode' => 'de']), 'Translation of May');
+    $this->assertEquals('Juni', t('June', [], ['langcode' => 'de']), 'Translation of June');
+    $this->assertEquals('Montag', t('Monday', [], ['langcode' => 'de']), 'Translation of Monday');
+    // cSpell:enable
+  }
+
+  /**
+   * Tests translation import from local sources.
+   *
+   * Test conditions:
+   *  - Source: local files only
+   *  - Import overwrite: all existing translations
+   */
+  public function testUpdateImportSourceLocal(): void {
+    $config = $this->config('locale.settings');
+
+    // Build the test environment.
+    $this->setTranslationFiles();
+    $this->setCurrentTranslations();
+    $config->set('translation.default_filename', '%project-%version.%language._po');
+
+    // Set the update conditions for this test.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_LOCAL,
+      'overwrite' => LOCALE_TRANSLATION_OVERWRITE_ALL,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Execute the translation update.
+    $this->drupalGet('admin/reports/translations/check');
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check if the translation has been updated, using the status cache.
+    $status = locale_translation_get_status();
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_one']['de']->type, 'Translation of contrib_module_one found');
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_two']['de']->type, 'Translation of contrib_module_two found');
+    $this->assertEquals(LOCALE_TRANSLATION_CURRENT, $status['contrib_module_three']['de']->type, 'Translation of contrib_module_three found');
+
+    // Check the new translation status.
+    // The static cache needs to be flushed first to get the most recent data
+    // from the database. The function was called earlier during this test.
+    drupal_static_reset('locale_translation_get_file_history');
+    $history = locale_translation_get_file_history();
+    // Verify that the translation of contrib_module_one is imported.
+    $this->assertGreaterThanOrEqual($this->timestampMedium, $history['contrib_module_one']['de']->timestamp);
+    $this->assertEquals($this->timestampMedium, $history['contrib_module_one']['de']->last_checked, 'Translation of contrib_module_one is updated');
+    $this->assertEquals($this->timestampNew, $history['contrib_module_two']['de']->timestamp, 'Translation of contrib_module_two is imported');
+    // Verify that the translation of contrib_module_two is updated.
+    $this->assertGreaterThanOrEqual($this->timestampNow, $history['contrib_module_two']['de']->last_checked);
+    $this->assertEquals($this->timestampMedium, $history['contrib_module_three']['de']->timestamp, 'Translation of contrib_module_three is not imported');
+    $this->assertEquals($this->timestampMedium, $history['contrib_module_three']['de']->last_checked, 'Translation of contrib_module_three is not updated');
+
+    // Check whether existing translations have (not) been overwritten.
+    // cSpell:disable
+    $this->assertEquals('Januar_customized', t('January', [], ['langcode' => 'de']), 'Translation of January');
+    $this->assertEquals('Februar_2', t('February', [], ['langcode' => 'de']), 'Translation of February');
+    $this->assertEquals('Marz_2', t('March', [], ['langcode' => 'de']), 'Translation of March');
+    $this->assertEquals('April_2', t('April', [], ['langcode' => 'de']), 'Translation of April');
+    $this->assertEquals('Mai_customized', t('May', [], ['langcode' => 'de']), 'Translation of May');
+    $this->assertEquals('Juni', t('June', [], ['langcode' => 'de']), 'Translation of June');
+    $this->assertEquals('Montag', t('Monday', [], ['langcode' => 'de']), 'Translation of Monday');
+    // cSpell:enable
+  }
+
+  /**
+   * Tests translation import and only overwrite non-customized translations.
+   *
+   * Test conditions:
+   *  - Source: remote and local files
+   *  - Import overwrite: only overwrite non-customized translations
+   */
+  public function testUpdateImportModeNonCustomized(): void {
+    $config = $this->config('locale.settings');
+
+    // Build the test environment.
+    $this->setTranslationFiles();
+    $this->setCurrentTranslations();
+    $config->set('translation.default_filename', '%project-%version.%language._po');
+
+    // Set the test conditions.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_REMOTE_AND_LOCAL,
+      'overwrite' => LOCALE_TRANSLATION_OVERWRITE_NON_CUSTOMIZED,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Execute translation update.
+    $this->drupalGet('admin/reports/translations/check');
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check whether existing translations have (not) been overwritten.
+    // cSpell:disable
+    $this->assertEquals('Januar_customized', t('January', [], ['langcode' => 'de']), 'Translation of January');
+    $this->assertEquals('Februar_customized', t('February', [], ['langcode' => 'de']), 'Translation of February');
+    $this->assertEquals('Marz_2', t('March', [], ['langcode' => 'de']), 'Translation of March');
+    $this->assertEquals('April_2', t('April', [], ['langcode' => 'de']), 'Translation of April');
+    $this->assertEquals('Mai_customized', t('May', [], ['langcode' => 'de']), 'Translation of May');
+    $this->assertEquals('Juni', t('June', [], ['langcode' => 'de']), 'Translation of June');
+    $this->assertEquals('Montag', t('Monday', [], ['langcode' => 'de']), 'Translation of Monday');
+    // cSpell:enable
+  }
+
+  /**
+   * Tests translation import and don't overwrite any translation.
+   *
+   * Test conditions:
+   *  - Source: remote and local files
+   *  - Import overwrite: don't overwrite any existing translation
+   */
+  public function testUpdateImportModeNone(): void {
+    $config = $this->config('locale.settings');
+
+    // Build the test environment.
+    $this->setTranslationFiles();
+    $this->setCurrentTranslations();
+    $config->set('translation.default_filename', '%project-%version.%language._po');
+
+    // Set the test conditions.
+    $edit = [
+      'use_source' => LOCALE_TRANSLATION_USE_SOURCE_REMOTE_AND_LOCAL,
+      'overwrite' => LOCALE_TRANSLATION_OVERWRITE_NONE,
+    ];
+    $this->drupalGet('admin/config/regional/translate/settings');
+    $this->submitForm($edit, 'Save configuration');
+
+    // Execute translation update.
+    $this->drupalGet('admin/reports/translations/check');
+    $this->drupalGet('admin/reports/translations');
+    $this->submitForm([], 'Update translations');
+
+    // Check whether existing translations have (not) been overwritten.
+    // cSpell:disable
+    $this->assertTranslation('January', 'Januar_customized', 'de');
+    $this->assertTranslation('February', 'Februar_customized', 'de');
+    $this->assertTranslation('March', 'Marz', 'de');
+    $this->assertTranslation('April', 'April_2', 'de');
+    $this->assertTranslation('May', 'Mai_customized', 'de');
+    $this->assertTranslation('June', 'Juni', 'de');
+    $this->assertTranslation('Monday', 'Montag', 'de');
+    // cSpell:enable
+  }
+
+  /**
+   * Tests automatic translation import when a module is enabled.
+   */
+  public function testEnableUninstallModule(): void {
+    // Make the hidden test modules look like a normal custom module.
+    \Drupal::state()->set('locale.test_system_info_alter', TRUE);
+
+    // Check if there is no translation yet.
+    $this->assertTranslation('Tuesday', '', 'de');
+
+    // Enable a module.
+    $edit = [
+      'modules[locale_test_translate][enable]' => 'locale_test_translate',
+    ];
+    $this->drupalGet('admin/modules');
+    $this->submitForm($edit, 'Install');
+
+    // Check if translations have been imported.
+    $this->assertSession()->pageTextContains("One translation file imported. 7 translations were added, 0 translations were updated and 0 translations were removed.");
+    // cSpell:disable-next-line
+    $this->assertTranslation('Tuesday', 'Dienstag', 'de');
+
+    $edit = [
+      'uninstall[locale_test_translate]' => 1,
+    ];
+    $this->drupalGet('admin/modules/uninstall');
+    $this->submitForm($edit, 'Uninstall');
+    $this->submitForm([], 'Uninstall');
+
+    // Check if the file data is removed from the database.
+    $history = locale_translation_get_file_history();
+    $this->assertFalse(isset($history['locale_test_translate']), 'Project removed from the file history');
+    $projects = locale_translation_get_projects();
+    $this->assertFalse(isset($projects['locale_test_translate']), 'Project removed from the project list');
+  }
+
+  /**
+   * Tests automatic translation import when a language is added.
+   *
+   * When a language is added, the system will check for translations files of
+   * enabled modules and will import them. When a language is removed the system
+   * will remove all translations of that language from the database.
+   */
+  public function testEnableLanguage(): void {
+    // Make the hidden test modules look like a normal custom module.
+    \Drupal::state()->set('locale.test_system_info_alter', TRUE);
+
+    // Enable a module.
+    $edit = [
+      'modules[locale_test_translate][enable]' => 'locale_test_translate',
+    ];
+    $this->drupalGet('admin/modules');
+    $this->submitForm($edit, 'Install');
+
+    // Check if there is no Dutch translation yet.
+    $this->assertTranslation('Extraday', '', 'nl');
+    // cSpell:disable-next-line
+    $this->assertTranslation('Tuesday', 'Dienstag', 'de');
+
+    // Add a language.
+    $edit = [
+      'predefined_langcode' => 'nl',
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add language');
+
+    // Check if the right number of translations are added.
+    $this->assertSession()->pageTextContains("One translation file imported. 8 translations were added, 0 translations were updated and 0 translations were removed.");
+    // cSpell:disable-next-line
+    $this->assertTranslation('Extra day', 'extra dag', 'nl');
+
+    // Check if the language data is added to the database.
+    $connection = Database::getConnection();
+    $result = $connection->select('locale_file', 'lf')
+      ->fields('lf', ['project'])
+      ->condition('langcode', 'nl')
+      ->execute()
+      ->fetchField();
+    $this->assertNotEmpty($result, 'Files added to file history');
+
+    // Remove a language.
+    $this->drupalGet('admin/config/regional/language/delete/nl');
+    $this->submitForm([], 'Delete');
+
+    // Check if the language data is removed from the database.
+    $result = $connection->select('locale_file', 'lf')
+      ->fields('lf', ['project'])
+      ->condition('langcode', 'nl')
+      ->execute()
+      ->fetchField();
+    $this->assertFalse($result, 'Files removed from file history');
+
+    // Check that the Dutch translation is gone.
+    $this->assertTranslation('Extra day', '', 'nl');
+    // cSpell:disable-next-line
+    $this->assertTranslation('Tuesday', 'Dienstag', 'de');
+  }
+
+  /**
+   * Tests automatic translation import when a custom language is added.
+   */
+  public function testEnableCustomLanguage(): void {
+    // Make the hidden test modules look like a normal custom module.
+    \Drupal::state()->set('locale.test_system_info_alter', TRUE);
+
+    // Enable a module.
+    $edit = [
+      'modules[locale_test_translate][enable]' => 'locale_test_translate',
+    ];
+    $this->drupalGet('admin/modules');
+    $this->submitForm($edit, 'Install');
+
+    // Create a custom language with language code 'xx' and a random
+    // name.
+    $langcode = 'xx';
+    $name = $this->randomMachineName(16);
+    $edit = [
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'label' => $name,
+      'direction' => LanguageInterface::DIRECTION_LTR,
+    ];
+    $this->drupalGet('admin/config/regional/language/add');
+    $this->submitForm($edit, 'Add custom language');
+
+    // Ensure the translation file is automatically imported when the language
+    // was added.
+    $this->assertSession()->pageTextContains('One translation file imported.');
+    $this->assertSession()->pageTextContains('One translation string was skipped because of disallowed or malformed HTML');
+
+    // Ensure the strings were successfully imported.
+    $search = [
+      'string' => 'lundi',
+      'langcode' => $langcode,
+      'translation' => 'translated',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextNotContains('No strings available.');
+
+    // Ensure the multiline string was imported.
+    $search = [
+      'string' => 'Source string for multiline translation',
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('Multiline translation string to make sure that import works with it.');
+
+    // Ensure 'Allowed HTML source string' was imported but the translation for
+    // 'Another allowed HTML source string' was not because it contains invalid
+    // HTML.
+    $search = [
+      'string' => 'HTML source string',
+      'langcode' => $langcode,
+      'translation' => 'all',
+    ];
+    $this->drupalGet('admin/config/regional/translate');
+    $this->submitForm($search, 'Filter');
+    $this->assertSession()->pageTextContains('Allowed HTML source string');
+    $this->assertSession()->pageTextNotContains('Another allowed HTML source string');
+  }
+
+}

core/modules/locale/tests/src/Functional/LocalesLocationAddIndexUpdateTest.php

@@ -0,0 +1,111 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Functional;
+
+use Drupal\FunctionalTests\Update\UpdatePathTestBase;
+
+/**
+ * Tests Locale update functions.
+ *
+ * @group locale
+ */
+class LocalesLocationAddIndexUpdateTest extends UpdatePathTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setDatabaseDumpFiles() {
+    $this->databaseDumpFiles[] = $this->root . '/core/modules/system/tests/fixtures/update/drupal-10.3.0.filled.standard.php.gz';
+  }
+
+  /**
+   * Tests locale_update_10300().
+   *
+   * @see locale_update_10300
+   */
+  public function testIndex(): void {
+    $this->assertFalse(\Drupal::database()
+      ->schema()
+      ->indexExists('locales_location', 'type_name'));
+
+    // Run updates and test them.
+    $this->runUpdates();
+
+    $this->assertTrue(\Drupal::database()
+      ->schema()
+      ->indexExists('locales_location', 'type_name'));
+  }
+
+  /**
+   * Tests locale_update_10300().
+   *
+   * @see locale_update_10300
+   */
+  public function testExistingIndex(): void {
+    $spec = [];
+    $spec['locales_location'] = [
+      'description' => 'Location information for source strings.',
+      'fields' => [
+        'lid' => [
+          'type' => 'serial',
+          'not null' => TRUE,
+          'description' => 'Unique identifier of this location.',
+        ],
+        'sid' => [
+          'type' => 'int',
+          'not null' => TRUE,
+          'description' => 'Unique identifier of this string.',
+        ],
+        'type' => [
+          'type' => 'varchar_ascii',
+          'length' => 50,
+          'not null' => TRUE,
+          'default' => '',
+          'description' => 'The location type (file, config, path, etc).',
+        ],
+        'name' => [
+          'type' => 'varchar',
+          'length' => 255,
+          'not null' => TRUE,
+          'default' => '',
+          'description' => 'Type dependent location information (file name, path, etc).',
+        ],
+        'version' => [
+          'type' => 'varchar_ascii',
+          'length' => 20,
+          'not null' => TRUE,
+          'default' => 'none',
+          'description' => 'Version of Drupal where the location was found.',
+        ],
+      ],
+      'primary key' => ['lid'],
+      'foreign keys' => [
+        'locales_source' => [
+          'table' => 'locales_source',
+          'columns' => ['sid' => 'lid'],
+        ],
+      ],
+      'indexes' => [
+        'string_type' => ['sid', 'type'],
+        'type_name' => ['type', 'name'],
+      ],
+    ];
+    \Drupal::database()->schema()->addIndex('locales_location', 'type_name', ['type', 'name', 'sid'], $spec['locales_location']);
+
+    // Run updates and test them.
+    $this->runUpdates();
+
+    // Ensure the update runs successfully even if an index existed prior to
+    // the update.
+    $schema = \Drupal::database()->schema();
+    $this->assertTrue($schema->indexExists('locales_location', 'type_name'));
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleBuildTest.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests building the translatable project information.
+ *
+ * @group locale
+ */
+class LocaleBuildTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'locale',
+    'locale_test',
+    'system',
+  ];
+
+  /**
+   * Checks if a list of translatable projects gets built.
+   */
+  public function testBuildProjects(): void {
+    $this->container->get('module_handler')->loadInclude('locale', 'compare.inc');
+    /** @var \Drupal\Core\Extension\ExtensionList $module_list */
+    $module_list = \Drupal::service('extension.list.module');
+
+    // Make the test modules look like a normal custom module. I.e. make the
+    // modules not hidden. locale_test_system_info_alter() modifies the project
+    // info of the locale_test and locale_test_translate modules.
+    \Drupal::state()->set('locale.test_system_info_alter', TRUE);
+
+    // Confirm the project name and core value before the module is altered.
+    $projects = locale_translation_build_projects();
+    $this->assertSame('locale_test', $projects['locale_test']->name);
+    $this->assertSame('all', $projects['locale_test']->core);
+
+    $projects['locale_test']->langcode = 'de';
+    $this->assertSame('/all/locale_test/locale_test-1.2.de.po', locale_translation_build_server_pattern($projects['locale_test'], '/%core/%project/%project-%version.%language.po'));
+
+    // Alter both the name and core value of the project.
+    \Drupal::state()->set('locale.test_system_info_alter_name_core', TRUE);
+    drupal_static_reset('locale_translation_project_list');
+    $module_list->reset();
+
+    // Confirm the name and core value are changed in $module->info.
+    $module = $module_list->get('locale_test');
+    $this->assertSame('locale_test_alter', $module->info['name']);
+    $this->assertSame('8.6.7', $module->info['core']);
+    $this->assertSame('locale_test', $module->getName());
+
+    // Confirm the name and core value are not changed in the project.
+    $projects = locale_translation_build_projects();
+    $this->assertSame('locale_test', $projects['locale_test']->name);
+    $this->assertSame('all', $projects['locale_test']->core);
+
+    $projects['locale_test']->langcode = 'de';
+    $this->assertSame('/all/locale_test/locale_test-1.2.de.po', locale_translation_build_server_pattern($projects['locale_test'], '/%core/%project/%project-%version.%language.po'));
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleConfigManagerTest.php

@@ -0,0 +1,131 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\block\Entity\Block;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests that the locale config manager operates correctly.
+ *
+ * @group locale
+ */
+class LocaleConfigManagerTest extends KernelTestBase {
+
+  /**
+   * A list of modules to install for this test.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    'system',
+    'language',
+    'locale',
+    'locale_test',
+    'block',
+  ];
+
+  /**
+   * This test creates simple config on the fly breaking schema checking.
+   *
+   * @var bool
+   */
+  protected $strictConfigSchema = FALSE;
+
+  /**
+   * Tests hasTranslation().
+   */
+  public function testHasTranslation(): void {
+    $this->installSchema('locale', ['locales_location', 'locales_source', 'locales_target']);
+    $this->installConfig(['locale_test']);
+    $locale_config_manager = \Drupal::service('locale.config_manager');
+
+    $language = ConfigurableLanguage::createFromLangcode('de');
+    $language->save();
+    $result = $locale_config_manager->hasTranslation('locale_test.no_translation', $language->getId());
+    $this->assertFalse($result, 'There is no translation for locale_test.no_translation configuration.');
+
+    $result = $locale_config_manager->hasTranslation('locale_test.translation', $language->getId());
+    $this->assertTrue($result, 'There is a translation for locale_test.translation configuration.');
+  }
+
+  /**
+   * Tests getStringTranslation().
+   */
+  public function testGetStringTranslation(): void {
+    $this->installSchema('locale', ['locales_location', 'locales_source', 'locales_target']);
+    $this->installConfig(['locale_test']);
+
+    $locale_config_manager = \Drupal::service('locale.config_manager');
+
+    $language = ConfigurableLanguage::createFromLangcode('de');
+    $language->save();
+
+    $translation_before = $locale_config_manager->getStringTranslation('locale_test.no_translation', $language->getId(), 'Test', '');
+    $this->assertTrue($translation_before->isNew());
+    $translation_before->setString('translation')->save();
+
+    $translation_after = $locale_config_manager->getStringTranslation('locale_test.no_translation', $language->getId(), 'Test', '');
+    $this->assertFalse($translation_after->isNew());
+    $translation_after->setString('updated_translation')->save();
+  }
+
+  /**
+   * Tests getDefaultConfigLangcode().
+   */
+  public function testGetDefaultConfigLangcode(): void {
+    // Install the Language module's configuration so we can use the
+    // module_installer service.
+    $this->installConfig(['language']);
+    $this->assertNull(\Drupal::service('locale.config_manager')->getDefaultConfigLangcode('locale_test_translate.settings'), 'Before installing a module the locale config manager can not access the shipped configuration.');
+    \Drupal::service('module_installer')->install(['locale_test_translate']);
+    $this->assertEquals('en', \Drupal::service('locale.config_manager')->getDefaultConfigLangcode('locale_test_translate.settings'), 'After installing a module the locale config manager can get the shipped configuration langcode.');
+
+    $simple_config = \Drupal::configFactory()->getEditable('locale_test_translate.simple_config_extra');
+    $simple_config->set('foo', 'bar')->save();
+    $this->assertNull(\Drupal::service('locale.config_manager')->getDefaultConfigLangcode($simple_config->getName()), 'Simple config created through the API is not treated as shipped configuration.');
+
+    $block = Block::create([
+      'id' => 'test_default_config',
+      'theme' => 'stark',
+      'status' => TRUE,
+      'region' => 'content',
+      'plugin' => 'local_tasks_block',
+      'settings' => [
+        'id' => 'local_tasks_block',
+        'label' => $this->randomMachineName(),
+        'provider' => 'core',
+        'label_display' => FALSE,
+        'primary' => TRUE,
+        'secondary' => TRUE,
+      ],
+    ]);
+    $block->save();
+
+    // Install the theme after creating the block as installing the theme will
+    // install the block provided by the locale_test module.
+    \Drupal::service('theme_installer')->install(['stark']);
+
+    // The test_default_config block provided by the locale_test module will not
+    // be installed because a block with the same ID already exists.
+    $this->installConfig(['locale_test']);
+    $this->assertNull(\Drupal::service('locale.config_manager')->getDefaultConfigLangcode('block.block.test_default_config'), 'The block.block.test_default_config is not shipped configuration.');
+    // Delete the block so we can install the one provided by the locale_test
+    // module.
+    $block->delete();
+    $this->installConfig(['locale_test']);
+    $this->assertEquals('en', \Drupal::service('locale.config_manager')->getDefaultConfigLangcode('block.block.test_default_config'), 'The block.block.test_default_config is shipped configuration.');
+
+    // Test the special case for configurable_language config entities.
+    $fr_language = ConfigurableLanguage::createFromLangcode('fr');
+    $fr_language->save();
+    $this->assertEquals('en', \Drupal::service('locale.config_manager')->getDefaultConfigLangcode('language.entity.fr'), 'The language.entity.fr is treated as shipped configuration because it is a configurable_language config entity and in the standard language list.');
+    $custom_language = ConfigurableLanguage::createFromLangcode('custom');
+    $custom_language->save();
+    $this->assertNull(\Drupal::service('locale.config_manager')->getDefaultConfigLangcode('language.entity.custom'), 'The language.entity.custom is not shipped configuration because it is not in the standard language list.');
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleConfigSubscriberForeignTest.php

@@ -0,0 +1,170 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Language\Language;
+use Drupal\language\Entity\ConfigurableLanguage;
+
+/**
+ * Tests default configuration handling with a foreign default language.
+ *
+ * @group locale
+ */
+class LocaleConfigSubscriberForeignTest extends LocaleConfigSubscriberTest {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function register(ContainerBuilder $container) {
+    parent::register($container);
+
+    $language = Language::$defaultValues;
+    $language['id'] = 'hu';
+    $language['name'] = 'Hungarian';
+    $container->setParameter('language.default_values', $language);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUpLanguages() {
+    parent::setUpLanguages();
+    ConfigurableLanguage::createFromLangcode('hu')->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUpLocale() {
+    parent::setUpLocale();
+    $this->setUpTranslation('locale_test.translation', 'test', 'English test', 'Hungarian test', 'hu', TRUE);
+  }
+
+  /**
+   * Tests that the language of default configuration was updated.
+   */
+  public function testDefaultConfigLanguage(): void {
+    $this->assertEquals('hu', $this->configFactory->getEditable('locale_test.no_translation')->get('langcode'));
+    $this->assertEquals('hu', $this->configFactory->getEditable('locale_test.translation')->get('langcode'));
+    $this->assertEquals('Hungarian test', $this->configFactory->getEditable('locale_test.translation')->get('test'));
+  }
+
+  /**
+   * Tests creating translations of shipped configuration.
+   */
+  public function testCreateActiveTranslation(): void {
+    $config_name = 'locale_test.no_translation';
+    $this->saveLanguageActive($config_name, 'test', 'Test (Hungarian)', 'hu');
+    $this->assertTranslation($config_name, 'Test (Hungarian)', 'hu');
+  }
+
+  /**
+   * Tests importing community translations of shipped configuration.
+   */
+  public function testLocaleCreateActiveTranslation(): void {
+    $config_name = 'locale_test.no_translation';
+    $this->saveLocaleTranslationData($config_name, 'test', 'Test', 'Test (Hungarian)', 'hu', TRUE);
+    $this->assertTranslation($config_name, 'Test (Hungarian)', 'hu', FALSE);
+  }
+
+  /**
+   * Tests updating translations of shipped configuration.
+   */
+  public function testUpdateActiveTranslation(): void {
+    $config_name = 'locale_test.translation';
+    $this->saveLanguageActive($config_name, 'test', 'Updated Hungarian test', 'hu');
+    $this->assertTranslation($config_name, 'Updated Hungarian test', 'hu');
+  }
+
+  /**
+   * Tests updating community translations of shipped configuration.
+   */
+  public function testLocaleUpdateActiveTranslation(): void {
+    $config_name = 'locale_test.translation';
+    $this->saveLocaleTranslationData($config_name, 'test', 'English test', 'Updated Hungarian test', 'hu', TRUE);
+    $this->assertTranslation($config_name, 'Updated Hungarian test', 'hu', FALSE);
+  }
+
+  /**
+   * Tests deleting a translation override.
+   */
+  public function testDeleteTranslation(): void {
+    $config_name = 'locale_test.translation';
+    $this->deleteLanguageOverride($config_name, 'test', 'English test', 'de');
+    // The German translation in this case will be forced to the Hungarian
+    // source so its not overwritten with locale data later.
+    $this->assertTranslation($config_name, 'Hungarian test', 'de');
+  }
+
+  /**
+   * Tests deleting translations of shipped configuration.
+   */
+  public function testDeleteActiveTranslation(): void {
+    $config_name = 'locale_test.translation';
+    $this->configFactory->getEditable($config_name)->delete();
+    // Deleting active configuration should not change the locale translation.
+    $this->assertTranslation($config_name, 'Hungarian test', 'hu', FALSE);
+  }
+
+  /**
+   * Tests deleting community translations of shipped configuration.
+   */
+  public function testLocaleDeleteActiveTranslation(): void {
+    $config_name = 'locale_test.translation';
+    $this->deleteLocaleTranslationData($config_name, 'test', 'English test', 'hu');
+    // Deleting the locale translation should not change active config.
+    $this->assertEquals('Hungarian test', $this->configFactory->getEditable($config_name)->get('test'));
+  }
+
+  /**
+   * Tests that adding English creates a translation override.
+   */
+  public function testEnglish(): void {
+    $config_name = 'locale_test.translation';
+    ConfigurableLanguage::createFromLangcode('en')->save();
+    // Adding a language on the UI would normally call updateConfigTranslations.
+    $this->localeConfigManager->updateConfigTranslations([$config_name], ['en']);
+    $this->assertConfigOverride($config_name, 'test', 'English test', 'en');
+
+    $this->configFactory->getEditable('locale.settings')->set('translate_english', TRUE)->save();
+    $this->saveLocaleTranslationData($config_name, 'test', 'English test', 'Updated English test', 'en');
+    $this->assertTranslation($config_name, 'Updated English test', 'en', FALSE);
+
+    $this->saveLanguageOverride($config_name, 'test', 'Updated English', 'en');
+    $this->assertTranslation($config_name, 'Updated English', 'en');
+
+    $this->deleteLocaleTranslationData($config_name, 'test', 'English test', 'en');
+    $this->assertNoConfigOverride($config_name, 'en');
+  }
+
+  /**
+   * Saves a language override.
+   *
+   * This will invoke LocaleConfigSubscriber through the event dispatcher. To
+   * make sure the configuration was persisted correctly, the configuration
+   * value is checked. Because LocaleConfigSubscriber temporarily disables the
+   * override state of the configuration factory we check that the correct value
+   * is restored afterwards.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $value
+   *   The configuration value to save.
+   * @param string $langcode
+   *   The language code.
+   */
+  protected function saveLanguageActive($config_name, $key, $value, $langcode) {
+    $this
+      ->configFactory
+      ->getEditable($config_name)
+      ->set($key, $value)
+      ->save();
+    $this->assertActiveConfig($config_name, $key, $value, $langcode);
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleConfigSubscriberTest.php

@@ -0,0 +1,497 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\locale\StringInterface;
+use Drupal\locale\TranslationString;
+
+/**
+ * Tests that shipped configuration translations are updated correctly.
+ *
+ * @group locale
+ */
+class LocaleConfigSubscriberTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['language', 'locale', 'system', 'locale_test'];
+
+  /**
+   * The configurable language manager used in this test.
+   *
+   * @var \Drupal\language\ConfigurableLanguageManagerInterface
+   */
+  protected $languageManager;
+
+  /**
+   * The configuration factory used in this test.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The string storage used in this test.
+   *
+   * @var \Drupal\locale\StringStorageInterface
+   */
+  protected $stringStorage;
+
+  /**
+   * The locale configuration manager used in this test.
+   *
+   * @var \Drupal\locale\LocaleConfigManager
+   */
+  protected $localeConfigManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->setUpDefaultLanguage();
+
+    $this->installSchema('locale', ['locales_source', 'locales_target', 'locales_location']);
+
+    $this->setupLanguages();
+
+    $this->installConfig(['locale_test']);
+    // Simulate this hook invoked which would happen if in a non-kernel test
+    // or normal environment.
+    // @see locale_modules_installed()
+    // @see locale_system_update()
+    locale_system_set_config_langcodes();
+    $langcodes = array_keys(\Drupal::languageManager()->getLanguages());
+    $locale_config_manager = \Drupal::service('locale.config_manager');
+    $names = $locale_config_manager->getComponentNames();
+    $locale_config_manager->updateConfigTranslations($names, $langcodes);
+
+    $this->configFactory = $this->container->get('config.factory');
+    $this->stringStorage = $this->container->get('locale.storage');
+    $this->localeConfigManager = $this->container->get('locale.config_manager');
+    $this->languageManager = $this->container->get('language_manager');
+
+    $this->setUpLocale();
+  }
+
+  /**
+   * Sets up default language for this test.
+   */
+  protected function setUpDefaultLanguage() {
+    // Keep the default English.
+  }
+
+  /**
+   * Sets up languages needed for this test.
+   */
+  protected function setUpLanguages() {
+    ConfigurableLanguage::createFromLangcode('de')->save();
+  }
+
+  /**
+   * Sets up the locale storage strings to be in line with configuration.
+   */
+  protected function setUpLocale() {
+    // Set up the locale database the same way we have in the config samples.
+    $this->setUpNoTranslation('locale_test.no_translation', 'test', 'Test', 'de');
+    $this->setUpTranslation('locale_test.translation', 'test', 'English test', 'German test', 'de');
+    $this->setUpTranslation('locale_test.translation_multiple', 'test', 'English test', 'German test', 'de');
+  }
+
+  /**
+   * Tests creating translations of shipped configuration.
+   */
+  public function testCreateTranslation(): void {
+    $config_name = 'locale_test.no_translation';
+
+    $this->saveLanguageOverride($config_name, 'test', 'Test (German)', 'de');
+    $this->assertTranslation($config_name, 'Test (German)', 'de');
+  }
+
+  /**
+   * Tests creating translations configuration with multi value settings.
+   */
+  public function testCreateTranslationMultiValue(): void {
+    $config_name = 'locale_test.translation_multiple';
+
+    $this->saveLanguageOverride($config_name, 'test_multiple', ['string' => 'String (German)', 'another_string' => 'Another string (German)'], 'de');
+    $this->saveLanguageOverride($config_name, 'test_after_multiple', ['string' => 'After string (German)', 'another_string' => 'After another string (German)'], 'de');
+    $strings = $this->stringStorage->getTranslations([
+      'type' => 'configuration',
+      'name' => $config_name,
+      'language' => 'de',
+      'translated' => TRUE,
+    ]);
+    $this->assertCount(5, $strings);
+  }
+
+  /**
+   * Tests importing community translations of shipped configuration.
+   */
+  public function testLocaleCreateTranslation(): void {
+    $config_name = 'locale_test.no_translation';
+
+    $this->saveLocaleTranslationData($config_name, 'test', 'Test', 'Test (German)', 'de');
+    $this->assertTranslation($config_name, 'Test (German)', 'de', FALSE);
+  }
+
+  /**
+   * Tests updating translations of shipped configuration.
+   */
+  public function testUpdateTranslation(): void {
+    $config_name = 'locale_test.translation';
+
+    $this->saveLanguageOverride($config_name, 'test', 'Updated German test', 'de');
+    $this->assertTranslation($config_name, 'Updated German test', 'de');
+  }
+
+  /**
+   * Tests updating community translations of shipped configuration.
+   */
+  public function testLocaleUpdateTranslation(): void {
+    $config_name = 'locale_test.translation';
+
+    $this->saveLocaleTranslationData($config_name, 'test', 'English test', 'Updated German test', 'de');
+    $this->assertTranslation($config_name, 'Updated German test', 'de', FALSE);
+  }
+
+  /**
+   * Tests deleting translations of shipped configuration.
+   */
+  public function testDeleteTranslation(): void {
+    $config_name = 'locale_test.translation';
+
+    $this->deleteLanguageOverride($config_name, 'test', 'English test', 'de');
+    // Instead of deleting the translation, we need to keep a translation with
+    // the source value and mark it as customized to prevent the deletion being
+    // reverted by importing community translations.
+    $this->assertTranslation($config_name, 'English test', 'de');
+  }
+
+  /**
+   * Tests deleting community translations of shipped configuration.
+   */
+  public function testLocaleDeleteTranslation(): void {
+    $config_name = 'locale_test.translation';
+
+    $this->deleteLocaleTranslationData($config_name, 'test', 'English test', 'de');
+    $this->assertNoTranslation($config_name, 'de');
+  }
+
+  /**
+   * Sets up a configuration string without a translation.
+   *
+   * The actual configuration is already available by installing locale_test
+   * module, as it is done in LocaleConfigSubscriberTest::setUp(). This sets up
+   * the necessary source string and verifies that everything is as expected to
+   * avoid false positives.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $source
+   *   The source string.
+   * @param string $langcode
+   *   The language code.
+   */
+  protected function setUpNoTranslation($config_name, $key, $source, $langcode) {
+    $this->localeConfigManager->updateConfigTranslations([$config_name], [$langcode]);
+    $this->assertNoConfigOverride($config_name, $key);
+    $this->assertNoTranslation($config_name, $langcode);
+  }
+
+  /**
+   * Sets up a configuration string with a translation.
+   *
+   * The actual configuration is already available by installing locale_test
+   * module, as it is done in LocaleConfigSubscriberTest::setUp(). This sets up
+   * the necessary source and translation strings and verifies that everything
+   * is as expected to avoid false positives.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $source
+   *   The source string.
+   * @param string $translation
+   *   The translation string.
+   * @param string $langcode
+   *   The language code.
+   * @param bool $is_active
+   *   Whether the update will affect the active configuration.
+   */
+  protected function setUpTranslation($config_name, $key, $source, $translation, $langcode, $is_active = FALSE) {
+    // Create source and translation strings for the configuration value and add
+    // the configuration name as a location. This would be performed by
+    // locale_translate_batch_import() invoking
+    // LocaleConfigManager::updateConfigTranslations() normally.
+    $this->localeConfigManager->reset();
+    $this->localeConfigManager
+      ->getStringTranslation($config_name, $langcode, $source, '')
+      ->setString($translation)
+      ->setCustomized(FALSE)
+      ->save();
+    $this->configFactory->reset($config_name);
+    $this->localeConfigManager->reset();
+    $this->localeConfigManager->updateConfigTranslations([$config_name], [$langcode]);
+
+    if ($is_active) {
+      $this->assertActiveConfig($config_name, $key, $translation, $langcode);
+    }
+    else {
+      $this->assertConfigOverride($config_name, $key, $translation, $langcode);
+    }
+    $this->assertTranslation($config_name, $translation, $langcode, FALSE);
+  }
+
+  /**
+   * Saves a language override.
+   *
+   * This will invoke LocaleConfigSubscriber through the event dispatcher. To
+   * make sure the configuration was persisted correctly, the configuration
+   * value is checked. Because LocaleConfigSubscriber temporarily disables the
+   * override state of the configuration factory we check that the correct value
+   * is restored afterwards.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string|array $value
+   *   The configuration value to save.
+   * @param string $langcode
+   *   The language code.
+   */
+  protected function saveLanguageOverride($config_name, $key, $value, $langcode) {
+    $translation_override = $this->languageManager
+      ->getLanguageConfigOverride($langcode, $config_name);
+    $translation_override
+      ->set($key, $value)
+      ->save();
+    $this->configFactory->reset($config_name);
+
+    $this->assertConfigOverride($config_name, $key, $value, $langcode);
+  }
+
+  /**
+   * Saves translation data from locale module.
+   *
+   * This will invoke LocaleConfigSubscriber through the event dispatcher. To
+   * make sure the configuration was persisted correctly, the configuration
+   * value is checked. Because LocaleConfigSubscriber temporarily disables the
+   * override state of the configuration factory we check that the correct value
+   * is restored afterwards.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $source
+   *   The source string.
+   * @param string $translation
+   *   The translation string to save.
+   * @param string $langcode
+   *   The language code.
+   * @param bool $is_active
+   *   Whether the update will affect the active configuration.
+   */
+  protected function saveLocaleTranslationData($config_name, $key, $source, $translation, $langcode, $is_active = FALSE) {
+    $this->localeConfigManager->reset();
+    $this->localeConfigManager
+      ->getStringTranslation($config_name, $langcode, $source, '')
+      ->setString($translation)
+      ->save();
+    $this->localeConfigManager->reset();
+    $this->localeConfigManager->updateConfigTranslations([$config_name], [$langcode]);
+    $this->configFactory->reset($config_name);
+
+    if ($is_active) {
+      $this->assertActiveConfig($config_name, $key, $translation, $langcode);
+    }
+    else {
+      $this->assertConfigOverride($config_name, $key, $translation, $langcode);
+    }
+  }
+
+  /**
+   * Deletes a language override.
+   *
+   * This will invoke LocaleConfigSubscriber through the event dispatcher. To
+   * make sure the configuration was persisted correctly, the configuration
+   * value is checked. Because LocaleConfigSubscriber temporarily disables the
+   * override state of the configuration factory we check that the correct value
+   * is restored afterwards.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $source_value
+   *   The source configuration value to verify the correct value is returned
+   *   from the configuration factory after the deletion.
+   * @param string $langcode
+   *   The language code.
+   */
+  protected function deleteLanguageOverride($config_name, $key, $source_value, $langcode) {
+    $translation_override = $this->languageManager
+      ->getLanguageConfigOverride($langcode, $config_name);
+    $translation_override
+      ->clear($key)
+      ->save();
+    $this->configFactory->reset($config_name);
+
+    $this->assertNoConfigOverride($config_name, $key);
+  }
+
+  /**
+   * Deletes translation data from locale module.
+   *
+   * This will invoke LocaleConfigSubscriber through the event dispatcher. To
+   * make sure the configuration was persisted correctly, the configuration
+   * value is checked. Because LocaleConfigSubscriber temporarily disables the
+   * override state of the configuration factory we check that the correct value
+   * is restored afterwards.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $source_value
+   *   The source configuration value to verify the correct value is returned
+   *   from the configuration factory after the deletion.
+   * @param string $langcode
+   *   The language code.
+   */
+  protected function deleteLocaleTranslationData($config_name, $key, $source_value, $langcode) {
+    $this->localeConfigManager
+      ->getStringTranslation($config_name, $langcode, $source_value, '')
+      ->delete();
+    $this->localeConfigManager->reset();
+    $this->localeConfigManager->updateConfigTranslations([$config_name], [$langcode]);
+    $this->configFactory->reset($config_name);
+
+    $this->assertNoConfigOverride($config_name, $key);
+  }
+
+  /**
+   * Ensures configuration override is not present anymore.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $langcode
+   *   The language code.
+   *
+   * @internal
+   */
+  protected function assertNoConfigOverride(string $config_name, string $langcode): void {
+    $config_langcode = $this->configFactory->getEditable($config_name)->get('langcode');
+    $override = $this->languageManager->getLanguageConfigOverride($langcode, $config_name);
+    $this->assertNotEquals($langcode, $config_langcode);
+    $this->assertTrue($override->isNew());
+  }
+
+  /**
+   * Ensures configuration was saved correctly.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string|array $value
+   *   The configuration value.
+   * @param string $langcode
+   *   The language code.
+   *
+   * @internal
+   */
+  protected function assertConfigOverride(string $config_name, string $key, $value, string $langcode): void {
+    $config_langcode = $this->configFactory->getEditable($config_name)->get('langcode');
+    $override = $this->languageManager->getLanguageConfigOverride($langcode, $config_name);
+    $this->assertNotEquals($langcode, $config_langcode);
+    $this->assertEquals($value, $override->get($key));
+  }
+
+  /**
+   * Ensures configuration was saved correctly.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $key
+   *   The configuration key.
+   * @param string $value
+   *   The configuration value.
+   * @param string $langcode
+   *   The language code.
+   *
+   * @internal
+   */
+  protected function assertActiveConfig(string $config_name, string $key, string $value, string $langcode): void {
+    $config = $this->configFactory->getEditable($config_name);
+    $this->assertEquals($langcode, $config->get('langcode'));
+    $this->assertSame($value, $config->get($key));
+  }
+
+  /**
+   * Ensures no translation exists.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string $langcode
+   *   The language code.
+   *
+   * @internal
+   */
+  protected function assertNoTranslation(string $config_name, string $langcode): void {
+    $strings = $this->stringStorage->getTranslations([
+      'type' => 'configuration',
+      'name' => $config_name,
+      'language' => $langcode,
+      'translated' => TRUE,
+    ]);
+    $this->assertSame([], $strings);
+  }
+
+  /**
+   * Asserts if a specific translation exists and its customization status.
+   *
+   * @param string $config_name
+   *   The configuration name.
+   * @param string|array $translation
+   *   The translation.
+   * @param string $langcode
+   *   The language code.
+   * @param bool $customized
+   *   (optional) Asserts if the translation is customized or not.
+   *
+   * @internal
+   */
+  protected function assertTranslation(string $config_name, $translation, string $langcode, bool $customized = TRUE): void {
+    // Make sure a string exists.
+    $strings = $this->stringStorage->getTranslations([
+      'type' => 'configuration',
+      'name' => $config_name,
+      'language' => $langcode,
+      'translated' => TRUE,
+    ]);
+    $this->assertCount(1, $strings);
+    $string = reset($strings);
+    $this->assertInstanceOf(StringInterface::class, $string);
+    /** @var \Drupal\locale\StringInterface $string */
+    $this->assertSame($translation, $string->getString());
+    $this->assertTrue($string->isTranslation());
+    $this->assertInstanceOf(TranslationString::class, $string);
+    /** @var \Drupal\locale\TranslationString $string */
+    // Make sure the string is marked as customized so that it does not get
+    // overridden when the string translations are updated.
+    $this->assertEquals($customized, (bool) $string->customized);
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleConfigurableLanguageManagerTest.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests that the configurable language manager and locale operate correctly.
+ *
+ * @group locale
+ */
+class LocaleConfigurableLanguageManagerTest extends KernelTestBase {
+
+  /**
+   * A list of modules to install for this test.
+   *
+   * @var array
+   */
+  protected static $modules = ['language', 'locale'];
+
+  public function testGetLanguages(): void {
+    $this->installSchema('locale', ['locales_source', 'locales_target', 'locales_location']);
+    $default_language = ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'default', 'weight' => 0]);
+    $default_language->save();
+
+    // Set new default language.
+    \Drupal::service('language.default')->set($default_language);
+    \Drupal::service('string_translation')->setDefaultLangcode($default_language->getId());
+
+    $languages = \Drupal::service('language_manager')->getLanguages(LanguageInterface::STATE_ALL);
+    $this->assertEquals(['default', 'und', 'zxx'], array_keys($languages));
+
+    $configurableLanguage = ConfigurableLanguage::create(['label' => $this->randomMachineName(), 'id' => 'test', 'weight' => 1]);
+    // Simulate a configuration sync by setting the flag otherwise the locked
+    // language weights would be updated whilst saving.
+    // @see \Drupal\language\Entity\ConfigurableLanguage::postSave()
+    $configurableLanguage->setSyncing(TRUE)->save();
+
+    $languages = \Drupal::service('language_manager')->getLanguages(LanguageInterface::STATE_ALL);
+    $this->assertEquals(['default', 'test', 'und', 'zxx'], array_keys($languages));
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleDefaultConfigStorageTest.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\Core\Config\NullStorage;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\locale\LocaleDefaultConfigStorage;
+
+/**
+ * @group locale
+ */
+class LocaleDefaultConfigStorageTest extends KernelTestBase {
+
+  protected static $modules = [
+    'language',
+    'locale',
+    'locale_test',
+    'locale_test_translate',
+  ];
+
+  public function testGetComponentNames(): void {
+    $storage = new LocaleDefaultConfigStorage(
+      new NullStorage(),
+      \Drupal::languageManager(),
+      'testing',
+    );
+
+    $expected = [
+      'locale_test.no_translation',
+      'locale_test.translation',
+      'locale_test.translation_multiple',
+      'locale_test_translate.settings',
+      'block.block.test_default_config',
+    ];
+    $actual = $storage->getComponentNames(
+      'module',
+      [
+        \Drupal::moduleHandler()->getModule('locale_test'),
+        \Drupal::moduleHandler()->getModule('locale_test_translate'),
+      ],
+    );
+    $this->assertSame($expected, $actual);
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleStringIsSafeTest.php

@@ -0,0 +1,95 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests locale translation safe string handling.
+ *
+ * @group locale
+ */
+class LocaleStringIsSafeTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['locale', 'locale_test'];
+
+  /**
+   * Tests for locale_string_is_safe().
+   */
+  public function testLocaleStringIsSafe(): void {
+    // Check a translatable string without HTML.
+    $string = 'Hello world!';
+    $result = locale_string_is_safe($string);
+    $this->assertTrue($result);
+
+    // Check a translatable string which includes trustable HTML.
+    $string = 'Hello <strong>world</strong>!';
+    $result = locale_string_is_safe($string);
+    $this->assertTrue($result);
+
+    // Check an untranslatable string which includes unsafe HTML (according to
+    // the locale_string_is_safe() function definition).
+    $string = 'Hello <img src="world.png" alt="world" />!';
+    $result = locale_string_is_safe($string);
+    $this->assertFalse($result);
+
+    // Check a translatable string which includes a token in an href attribute.
+    $string = 'Hi <a href="[current-user:url]">user</a>';
+    $result = locale_string_is_safe($string);
+    $this->assertTrue($result);
+  }
+
+  /**
+   * Tests if a translated and tokenized string is properly escaped by Twig.
+   *
+   * In each assert* call we add a new line at the expected result to match the
+   * newline at the end of the template file.
+   */
+  public function testLocalizedTokenizedString(): void {
+    $tests_to_do = [
+      1 => [
+        'original' => 'Go to the <a href="[locale_test:security_test1]">frontpage</a>',
+        'replaced' => 'Go to the &lt;a href=&quot;javascript:alert(&amp;#039;Hello!&amp;#039;);&quot;&gt;frontpage&lt;/a&gt;',
+      ],
+      2 => [
+        'original' => 'Hello <strong>[locale_test:security_test2]</strong>!',
+        'replaced' => 'Hello &lt;strong&gt;&amp;lt;script&amp;gt;alert(&amp;#039;Hello!&amp;#039;);&amp;lt;/script&amp;gt;&lt;/strong&gt;!',
+      ],
+    ];
+
+    foreach ($tests_to_do as $i => $test) {
+      $original_string = $test['original'];
+      $rendered_original_string = \Drupal::theme()->render('locale_test_tokenized', ['content' => $original_string]);
+      // Twig assumes that strings are unsafe so it escapes them, and so the
+      // original and the rendered version should be different.
+      $this->assertNotEquals(
+        $original_string . "\n",
+        $rendered_original_string,
+        'Security test ' . $i . ' before translation'
+      );
+
+      // Pass the original string to the t() function to get it marked as safe.
+      $safe_string = t($original_string);
+      $rendered_safe_string = \Drupal::theme()->render('locale_test_tokenized', ['content' => $safe_string]);
+      // t() function always marks the string as safe so it won't be escaped,
+      // and should be the same as the original.
+      $this->assertSame($original_string . "\n", (string) $rendered_safe_string, 'Security test ' . $i . ' after translation before token replacement');
+
+      // Replace tokens in the safe string to inject it with dangerous content.
+      // @see locale_test_tokens().
+      $unsafe_string = \Drupal::token()->replace($safe_string);
+      $rendered_unsafe_string = \Drupal::theme()->render('locale_test_tokenized', ['content' => $unsafe_string]);
+      // Token replacement changes the string so it is not marked as safe
+      // anymore. Check it is escaped the way we expect.
+      $this->assertEquals($test['replaced'] . "\n", $rendered_unsafe_string, 'Security test ' . $i . ' after translation  after token replacement');
+    }
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleStringTest.php

@@ -0,0 +1,245 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\locale\StringInterface;
+
+/**
+ * Tests the locale string storage, string objects and data API.
+ *
+ * @group locale
+ */
+class LocaleStringTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'language',
+    'locale',
+  ];
+
+  /**
+   * The locale storage.
+   *
+   * @var \Drupal\locale\StringStorageInterface
+   */
+  protected $storage;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // Add a default locale storage for all these tests.
+    $this->storage = $this->container->get('locale.storage');
+    // Create two languages: Spanish and German.
+    foreach (['es', 'de'] as $langcode) {
+      ConfigurableLanguage::createFromLangcode($langcode)->save();
+    }
+    $this->installSchema('locale', [
+      'locales_location',
+      'locales_source',
+      'locales_target',
+    ]);
+  }
+
+  /**
+   * Tests CRUD API.
+   */
+  public function testStringCrudApi(): void {
+    // Create source string.
+    $source = $this->buildSourceString()->save();
+    $this->assertNotEmpty($source->lid);
+
+    // Load strings by lid and source.
+    $string1 = $this->storage->findString(['lid' => $source->lid]);
+    $this->assertEquals($source, $string1);
+    $string2 = $this->storage->findString(['source' => $source->source, 'context' => $source->context]);
+    $this->assertEquals($source, $string2);
+    $string3 = $this->storage->findString(['source' => $source->source, 'context' => '']);
+    $this->assertNull($string3);
+
+    // Check version handling and updating.
+    $this->assertEquals('none', $source->version);
+    $string = $this->storage->findTranslation(['lid' => $source->lid]);
+    $this->assertEquals(\Drupal::VERSION, $string->version);
+
+    // Create translation and find it by lid and source.
+    $langcode = 'es';
+    $translation = $this->createTranslation($source, $langcode);
+    $this->assertEquals(LOCALE_NOT_CUSTOMIZED, $translation->customized);
+    $string1 = $this->storage->findTranslation(['language' => $langcode, 'lid' => $source->lid]);
+    $this->assertEquals($translation->translation, $string1->translation);
+    $string2 = $this->storage->findTranslation(['language' => $langcode, 'source' => $source->source, 'context' => $source->context]);
+    $this->assertEquals($translation->translation, $string2->translation);
+    $translation
+      ->setCustomized()
+      ->save();
+    $translation = $this->storage->findTranslation(['language' => $langcode, 'lid' => $source->lid]);
+    $this->assertEquals(LOCALE_CUSTOMIZED, $translation->customized);
+
+    // Delete translation.
+    $translation->delete();
+    $deleted = $this->storage->findTranslation(['language' => $langcode, 'lid' => $source->lid]);
+    $this->assertNull($deleted->translation);
+
+    // Create some translations and then delete string and all of its
+    // translations.
+    $lid = $source->lid;
+    $this->createAllTranslations($source);
+    $search = $this->storage->getTranslations(['lid' => $source->lid]);
+    $this->assertCount(3, $search);
+
+    $source->delete();
+    $string = $this->storage->findString(['lid' => $lid]);
+    $this->assertNull($string);
+    $deleted = $search = $this->storage->getTranslations(['lid' => $lid]);
+    $this->assertEmpty($deleted);
+
+    // Tests that locations of different types and arbitrary lengths can be
+    // added to a source string. Too long locations will be cut off.
+    $source_string = $this->buildSourceString();
+    $source_string->addLocation('javascript', $this->randomString(8));
+    $source_string->addLocation('configuration', $this->randomString(50));
+    $source_string->addLocation('code', $this->randomString(100));
+    $source_string->addLocation('path', $location = $this->randomString(300));
+    $source_string->save();
+
+    $rows = $this->container->get('database')->select('locales_location')
+      ->fields('locales_location')
+      ->condition('sid', $source_string->lid)
+      ->execute()
+      ->fetchAllAssoc('type');
+    $this->assertCount(4, $rows);
+    $this->assertEquals(substr($location, 0, 255), $rows['path']->name);
+  }
+
+  /**
+   * Tests Search API loading multiple objects.
+   */
+  public function testStringSearchApi(): void {
+    $language_count = 3;
+    // Strings 1 and 2 will have some common prefix.
+    // Source 1 will have all translations, not customized.
+    // Source 2 will have all translations, customized.
+    // Source 3 will have no translations.
+    $prefix = $this->randomMachineName(100);
+    $source1 = $this->buildSourceString(['source' => $prefix . $this->randomMachineName(100)])->save();
+    $source2 = $this->buildSourceString(['source' => $prefix . $this->randomMachineName(100)])->save();
+    $source3 = $this->buildSourceString()->save();
+
+    // Load all source strings.
+    $strings = $this->storage->getStrings([]);
+    $this->assertCount(3, $strings);
+    // Load all source strings matching a given string.
+    $filter_options['filters'] = ['source' => $prefix];
+    $strings = $this->storage->getStrings([], $filter_options);
+    $this->assertCount(2, $strings);
+
+    // Not customized translations.
+    $translate1 = $this->createAllTranslations($source1);
+    // Customized translations.
+    $this->createAllTranslations($source2, ['customized' => LOCALE_CUSTOMIZED]);
+    // Try quick search function with different field combinations.
+    $langcode = 'es';
+    $found = $this->storage->findTranslation(['language' => $langcode, 'source' => $source1->source, 'context' => $source1->context]);
+    $this->assertNotNull($found, 'Translation not found searching by source and context.');
+    $this->assertNotNull($found->language);
+    $this->assertNotNull($found->translation);
+    $this->assertFalse($found->isNew());
+    $this->assertEquals($translate1[$langcode]->translation, $found->translation);
+    // Now try a translation not found.
+    $found = $this->storage->findTranslation(['language' => $langcode, 'source' => $source3->source, 'context' => $source3->context]);
+    $this->assertNotNull($found);
+    $this->assertSame($source3->lid, $found->lid);
+    $this->assertNull($found->translation);
+    $this->assertTrue($found->isNew());
+
+    // Load all translations. For next queries we'll be loading only translated
+    // strings.
+    $translations = $this->storage->getTranslations(['translated' => TRUE]);
+    $this->assertCount(2 * $language_count, $translations);
+
+    // Load all customized translations.
+    $translations = $this->storage->getTranslations(['customized' => LOCALE_CUSTOMIZED, 'translated' => TRUE]);
+    $this->assertCount($language_count, $translations);
+
+    // Load all Spanish customized translations.
+    $translations = $this->storage->getTranslations(['language' => 'es', 'customized' => LOCALE_CUSTOMIZED, 'translated' => TRUE]);
+    $this->assertCount(1, $translations);
+
+    // Load all source strings without translation (1).
+    $translations = $this->storage->getStrings(['translated' => FALSE]);
+    $this->assertCount(1, $translations);
+
+    // Load Spanish translations using string filter.
+    $filter_options['filters'] = ['source' => $prefix];
+    $translations = $this->storage->getTranslations(['language' => 'es'], $filter_options);
+    $this->assertCount(2, $translations);
+  }
+
+  /**
+   * Creates random source string object.
+   *
+   * @param array $values
+   *   The values array.
+   *
+   * @return \Drupal\locale\StringInterface
+   *   A locale string.
+   */
+  protected function buildSourceString(array $values = []) {
+    return $this->storage->createString($values += [
+      'source' => $this->randomMachineName(100),
+      'context' => $this->randomMachineName(20),
+    ]);
+  }
+
+  /**
+   * Creates translations for source string and all languages.
+   *
+   * @param \Drupal\locale\StringInterface $source
+   *   The source string.
+   * @param array $values
+   *   The values array.
+   *
+   * @return array
+   *   Translation list.
+   */
+  protected function createAllTranslations(StringInterface $source, array $values = []) {
+    $list = [];
+    /** @var \Drupal\Core\Language\LanguageManagerInterface $language_manager */
+    $language_manager = $this->container->get('language_manager');
+    foreach ($language_manager->getLanguages() as $language) {
+      $list[$language->getId()] = $this->createTranslation($source, $language->getId(), $values);
+    }
+    return $list;
+  }
+
+  /**
+   * Creates single translation for source string.
+   *
+   * @param \Drupal\locale\StringInterface $source
+   *   The source string.
+   * @param string $langcode
+   *   The language code.
+   * @param array $values
+   *   The values array.
+   *
+   * @return \Drupal\locale\StringInterface
+   *   The translated string object.
+   */
+  protected function createTranslation(StringInterface $source, $langcode, array $values = []) {
+    return $this->storage->createTranslation($values + [
+      'lid' => $source->lid,
+      'language' => $langcode,
+      'translation' => $this->randomMachineName(100),
+    ])->save();
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleTranslationProjectsTest.php

@@ -0,0 +1,65 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests locale translation project handling.
+ *
+ * @group locale
+ */
+class LocaleTranslationProjectsTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['locale', 'locale_test', 'system'];
+
+  /**
+   * The module handler used in this test.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The locale project storage used in this test.
+   *
+   * @var \Drupal\locale\LocaleProjectStorageInterface
+   */
+  protected $projectStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->moduleHandler = $this->container->get('module_handler');
+    $this->projectStorage = $this->container->get('locale.project');
+    \Drupal::state()->set('locale.remove_core_project', TRUE);
+  }
+
+  /**
+   * Tests locale_translation_clear_cache_projects().
+   */
+  public function testLocaleTranslationClearCacheProjects(): void {
+    $this->moduleHandler->loadInclude('locale', 'inc', 'locale.translation');
+
+    $expected = [];
+    $this->assertSame($expected, locale_translation_get_projects());
+
+    $this->projectStorage->set('foo', []);
+    $expected['foo'] = new \stdClass();
+    $this->assertEquals($expected, locale_translation_get_projects());
+
+    $this->projectStorage->set('bar', []);
+    locale_translation_clear_cache_projects();
+    $expected['bar'] = new \stdClass();
+    $this->assertEquals($expected, locale_translation_get_projects());
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleTranslationTest.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\locale\LocaleTranslation;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\locale\LocaleTranslation
+ * @group locale
+ */
+class LocaleTranslationTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'locale',
+  ];
+
+  /**
+   * Tests that \Drupal\locale\LocaleTranslation is serializable.
+   */
+  public function testSerializable(): void {
+    $translation = $this->container->get('string_translator.locale.lookup');
+    $this->assertInstanceOf(LocaleTranslation::class, $translation);
+
+    // Prove that serialization and deserialization works without errors.
+    $this->assertNotNull($translation);
+    $unserialized = unserialize(serialize($translation));
+    $this->assertInstanceOf(LocaleTranslation::class, $unserialized);
+  }
+
+}

core/modules/locale/tests/src/Kernel/LocaleUpdateTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests for updating the interface translations of projects.
+ *
+ * @group locale
+ */
+class LocaleUpdateTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'locale',
+    'locale_test',
+    'system',
+  ];
+
+  /**
+   * Checks if a list of translatable projects gets build.
+   */
+  public function testUpdateProjects(): void {
+    $this->container->get('module_handler')->loadInclude('locale', 'compare.inc');
+
+    // Make the test modules look like a normal custom module. I.e. make the
+    // modules not hidden. locale_test_system_info_alter() modifies the project
+    // info of the locale_test and locale_test_translate modules.
+    \Drupal::state()->set('locale.test_system_info_alter', TRUE);
+
+    // Check if interface translation data is collected from hook_info.
+    $projects = locale_translation_project_list();
+    $this->assertArrayNotHasKey('locale_test_translate', $projects);
+    $this->assertEquals('core/modules/locale/test/test.%language.po', $projects['locale_test']['info']['interface translation server pattern']);
+    $this->assertEquals('locale_test', $projects['locale_test']['name']);
+  }
+
+}

core/modules/locale/tests/src/Kernel/Migrate/MigrateLocaleConfigsTest.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Kernel\Migrate;
+
+use Drupal\Tests\SchemaCheckTestTrait;
+use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
+
+/**
+ * Upgrade variables to locale.settings.yml.
+ *
+ * @group migrate_drupal_6
+ */
+class MigrateLocaleConfigsTest extends MigrateDrupal6TestBase {
+
+  use SchemaCheckTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['locale', 'language'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->executeMigration('locale_settings');
+  }
+
+  /**
+   * Tests migration of locale variables to locale.settings.yml.
+   */
+  public function testLocaleSettings(): void {
+    $config = $this->config('locale.settings');
+    $this->assertTrue($config->get('cache_strings'));
+    $this->assertSame('languages', $config->get('javascript.directory'));
+    $this->assertConfigSchema(\Drupal::service('config.typed'), 'locale.settings', $config->get());
+  }
+
+}

core/modules/locale/tests/src/Unit/LocaleLookupTest.php

@@ -0,0 +1,390 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit;
+
+use Drupal\Component\Gettext\PoItem;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\locale\LocaleLookup;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * @coversDefaultClass \Drupal\locale\LocaleLookup
+ * @group locale
+ */
+class LocaleLookupTest extends UnitTestCase {
+
+  /**
+   * A mocked storage to use when instantiating LocaleTranslation objects.
+   *
+   * @var \Drupal\locale\StringStorageInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $storage;
+
+  /**
+   * A mocked cache object.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $cache;
+
+  /**
+   * A mocked lock object.
+   *
+   * @var \Drupal\Core\Lock\LockBackendInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $lock;
+
+  /**
+   * A mocked user object built from AccountInterface.
+   *
+   * @var \Drupal\Core\Session\AccountInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $user;
+
+  /**
+   * A mocked config factory built with UnitTestCase::getConfigFactoryStub().
+   *
+   * @var \Drupal\Core\Config\ConfigFactory|\PHPUnit\Framework\MockObject\MockBuilder
+   */
+  protected $configFactory;
+
+  /**
+   * A mocked language manager built from LanguageManagerInterface.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $languageManager;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->storage = $this->createMock('Drupal\locale\StringStorageInterface');
+    $this->cache = $this->createMock('Drupal\Core\Cache\CacheBackendInterface');
+    $this->lock = $this->createMock('Drupal\Core\Lock\LockBackendInterface');
+    $this->lock->expects($this->never())
+      ->method($this->anything());
+
+    $this->user = $this->createMock('Drupal\Core\Session\AccountInterface');
+    $this->user->expects($this->any())
+      ->method('getRoles')
+      ->willReturn(['anonymous']);
+
+    $this->configFactory = $this->getConfigFactoryStub(['locale.settings' => ['cache_strings' => FALSE]]);
+
+    $this->languageManager = $this->createMock('Drupal\Core\Language\LanguageManagerInterface');
+    $this->requestStack = new RequestStack();
+
+    $container = new ContainerBuilder();
+    $container->set('current_user', $this->user);
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * Tests locale lookups without fallback.
+   *
+   * @covers ::resolveCacheMiss
+   */
+  public function testResolveCacheMissWithoutFallback(): void {
+    $args = [
+      'language' => 'en',
+      'source' => 'test',
+      'context' => 'irrelevant',
+    ];
+
+    $result = (object) [
+      'translation' => 'test',
+    ];
+
+    $this->cache->expects($this->once())
+      ->method('get')
+      ->with('locale:en:irrelevant:anonymous', FALSE);
+
+    $this->storage->expects($this->once())
+      ->method('findTranslation')
+      ->with($this->equalTo($args))
+      ->willReturn($result);
+
+    $locale_lookup = $this->getMockBuilder('Drupal\locale\LocaleLookup')
+      ->setConstructorArgs(['en', 'irrelevant', $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack])
+      ->onlyMethods(['persist'])
+      ->getMock();
+    $locale_lookup->expects($this->never())
+      ->method('persist');
+    $this->assertSame('test', $locale_lookup->get('test'));
+  }
+
+  /**
+   * Tests locale lookups with fallback.
+   *
+   * Note that context is irrelevant here. It is not used but it is required.
+   *
+   * @covers ::resolveCacheMiss
+   *
+   * @dataProvider resolveCacheMissWithFallbackProvider
+   */
+  public function testResolveCacheMissWithFallback($langcode, $string, $context, $expected): void {
+    // These are fake words!
+    // cSpell:disable
+    $translations = [
+      'en' => [
+        'test' => 'test',
+        'fake' => 'fake',
+        'missing pl' => 'missing pl',
+        'missing cs' => 'missing cs',
+        'missing both' => 'missing both',
+      ],
+      'pl' => [
+        'test' => 'test po polsku',
+        'fake' => 'ściema',
+        'missing cs' => 'zaginiony czech',
+      ],
+      'cs' => [
+        'test' => 'test v české',
+        'fake' => 'falešný',
+        'missing pl' => 'chybějící pl',
+      ],
+    ];
+    // cSpell:enable
+    $this->storage->expects($this->any())
+      ->method('findTranslation')
+      ->willReturnCallback(function ($argument) use ($translations) {
+        if (isset($translations[$argument['language']][$argument['source']])) {
+          return (object) ['translation' => $translations[$argument['language']][$argument['source']]];
+        }
+
+        return TRUE;
+      });
+
+    $this->languageManager->expects($this->any())
+      ->method('getFallbackCandidates')
+      ->willReturnCallback(function (array $context = []) {
+        switch ($context['langcode']) {
+          case 'pl':
+            return ['cs', 'en'];
+
+          case 'cs':
+            return ['en'];
+
+          default:
+            return [];
+        }
+      });
+
+    $this->cache->expects($this->once())
+      ->method('get')
+      ->with('locale:' . $langcode . ':' . $context . ':anonymous', FALSE);
+
+    $locale_lookup = new LocaleLookup($langcode, $context, $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack);
+    $this->assertSame($expected, $locale_lookup->get($string));
+  }
+
+  /**
+   * Provides test data for testResolveCacheMissWithFallback().
+   */
+  public static function resolveCacheMissWithFallbackProvider() {
+    // cSpell:disable
+    return [
+      ['cs', 'test', 'irrelevant', 'test v české'],
+      ['cs', 'fake', 'irrelevant', 'falešný'],
+      ['cs', 'missing pl', 'irrelevant', 'chybějící pl'],
+      ['cs', 'missing cs', 'irrelevant', 'missing cs'],
+      ['cs', 'missing both', 'irrelevant', 'missing both'],
+
+      // Testing PL with fallback to cs, en.
+      ['pl', 'test', 'irrelevant', 'test po polsku'],
+      ['pl', 'fake', 'irrelevant', 'ściema'],
+      ['pl', 'missing pl', 'irrelevant', 'chybějící pl'],
+      ['pl', 'missing cs', 'irrelevant', 'zaginiony czech'],
+      ['pl', 'missing both', 'irrelevant', 'missing both'],
+    ];
+    // cSpell:enable
+  }
+
+  /**
+   * Tests locale lookups with persistent tracking.
+   *
+   * @covers ::resolveCacheMiss
+   */
+  public function testResolveCacheMissWithPersist(): void {
+    $args = [
+      'language' => 'en',
+      'source' => 'test',
+      'context' => 'irrelevant',
+    ];
+
+    $result = (object) [
+      'translation' => 'test',
+    ];
+
+    $this->storage->expects($this->once())
+      ->method('findTranslation')
+      ->with($this->equalTo($args))
+      ->willReturn($result);
+
+    $this->configFactory = $this->getConfigFactoryStub(['locale.settings' => ['cache_strings' => TRUE]]);
+    $locale_lookup = $this->getMockBuilder('Drupal\locale\LocaleLookup')
+      ->setConstructorArgs(['en', 'irrelevant', $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack])
+      ->onlyMethods(['persist'])
+      ->getMock();
+    $locale_lookup->expects($this->once())
+      ->method('persist');
+
+    $this->assertSame('test', $locale_lookup->get('test'));
+  }
+
+  /**
+   * Tests locale lookups without a found translation.
+   *
+   * @covers ::resolveCacheMiss
+   */
+  public function testResolveCacheMissNoTranslation(): void {
+    $string = $this->createMock('Drupal\locale\StringInterface');
+    $string->expects($this->once())
+      ->method('addLocation')
+      ->willReturnSelf();
+    $this->storage->expects($this->once())
+      ->method('findTranslation')
+      ->willReturn(NULL);
+    $this->storage->expects($this->once())
+      ->method('createString')
+      ->willReturn($string);
+
+    $request = Request::create('/test');
+    $this->requestStack->push($request);
+
+    $locale_lookup = $this->getMockBuilder('Drupal\locale\LocaleLookup')
+      ->setConstructorArgs(['en', 'irrelevant', $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack])
+      ->onlyMethods(['persist'])
+      ->getMock();
+    $locale_lookup->expects($this->never())
+      ->method('persist');
+
+    $this->assertTrue($locale_lookup->get('test'));
+  }
+
+  /**
+   * Tests locale lookups with old plural style of translations.
+   *
+   * @param array $translations
+   *   The source with translations.
+   * @param string $langcode
+   *   The language code of translation string.
+   * @param string $string
+   *   The string for translation.
+   * @param bool $is_fix
+   *   The flag about expected fix translation.
+   *
+   * @covers ::resolveCacheMiss
+   * @dataProvider providerFixOldPluralTranslationProvider
+   */
+  public function testFixOldPluralStyleTranslations($translations, $langcode, $string, $is_fix): void {
+    $this->storage->expects($this->any())
+      ->method('findTranslation')
+      ->willReturnCallback(function ($argument) use ($translations) {
+        if (isset($translations[$argument['language']][$argument['source']])) {
+          return (object) ['translation' => $translations[$argument['language']][$argument['source']]];
+        }
+
+        return TRUE;
+      });
+    $this->languageManager->expects($this->any())
+      ->method('getFallbackCandidates')
+      ->willReturnCallback(function (array $context = []) {
+        switch ($context['langcode']) {
+          case 'by':
+            return ['ru'];
+        }
+      });
+    $this->cache->expects($this->once())
+      ->method('get')
+      ->with('locale:' . $langcode . '::anonymous', FALSE);
+
+    $locale_lookup = new LocaleLookup($langcode, '', $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack);
+    if ($is_fix) {
+      $this->assertStringNotContainsString('@count[2]', $locale_lookup->get($string));
+    }
+    else {
+      $this->assertStringContainsString('@count[2]', $locale_lookup->get($string));
+    }
+  }
+
+  /**
+   * Provides test data for testResolveCacheMissWithFallback().
+   */
+  public static function providerFixOldPluralTranslationProvider() {
+    $translations = [
+      'by' => [
+        'word1' => '@count[2] word-by',
+        'word2' => implode(PoItem::DELIMITER, ['word-by', '@count[2] word-by']),
+      ],
+      'ru' => [
+        'word3' => '@count[2] word-ru',
+        'word4' => implode(PoItem::DELIMITER, ['word-ru', '@count[2] word-ru']),
+      ],
+    ];
+    return [
+      'no-plural' => [$translations, 'by', 'word1', FALSE],
+      'no-plural from other language' => [$translations, 'by', 'word3', FALSE],
+      'plural' => [$translations, 'by', 'word2', TRUE],
+      'plural from other language' => [$translations, 'by', 'word4', TRUE],
+    ];
+  }
+
+  /**
+   * @covers ::getCid
+   *
+   * @dataProvider getCidProvider
+   */
+  public function testGetCid(array $roles, $expected): void {
+    $this->user = $this->createMock('Drupal\Core\Session\AccountInterface');
+    $this->user->expects($this->any())
+      ->method('getRoles')
+      ->willReturn($roles);
+
+    $container = new ContainerBuilder();
+    $container->set('current_user', $this->user);
+    \Drupal::setContainer($container);
+
+    $locale_lookup = $this->getMockBuilder('Drupal\locale\LocaleLookup')
+      ->setConstructorArgs(['en', 'irrelevant', $this->storage, $this->cache, $this->lock, $this->configFactory, $this->languageManager, $this->requestStack])
+      ->getMock();
+
+    $o = new \ReflectionObject($locale_lookup);
+    $method = $o->getMethod('getCid');
+    $cid = $method->invoke($locale_lookup, 'getCid');
+
+    $this->assertEquals($expected, $cid);
+  }
+
+  /**
+   * Provides test data for testGetCid().
+   */
+  public static function getCidProvider() {
+    return [
+      [
+        ['a'], 'locale:en:irrelevant:a',
+      ],
+      [
+        ['a', 'b'], 'locale:en:irrelevant:a:b',
+      ],
+      [
+        ['b', 'a'], 'locale:en:irrelevant:a:b',
+      ],
+    ];
+  }
+
+}

core/modules/locale/tests/src/Unit/LocaleProjectStorageTest.php

@@ -0,0 +1,101 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit;
+
+use Drupal\Core\KeyValueStore\KeyValueMemoryFactory;
+use Drupal\locale\LocaleProjectStorage;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\locale\LocaleProjectStorage
+ * @group locale
+ * @runTestsInSeparateProcesses
+ */
+class LocaleProjectStorageTest extends UnitTestCase {
+
+  /**
+   * @var \Drupal\locale\LocaleProjectStorage
+   */
+  private LocaleProjectStorage $projectStorage;
+
+  /**
+   * @var \Drupal\Core\KeyValueStore\KeyValueMemoryFactory
+   */
+  private KeyValueMemoryFactory $keyValueMemoryFactory;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->keyValueMemoryFactory = new KeyValueMemoryFactory();
+    $this->projectStorage = new LocaleProjectStorage($this->keyValueMemoryFactory);
+  }
+
+  /**
+   * Tests that projects are sorted by weight and key.
+   */
+  public function testSorting(): void {
+    // There are no projects.
+    $this->assertSame([], $this->projectStorage->getAll());
+
+    // Add project 'b'.
+    $this->projectStorage->set('b', ['name' => 'b']);
+    $this->assertSame(['b'], array_keys($this->projectStorage->getAll()));
+
+    // Add project 'c' and confirm alphabetical order.
+    $this->projectStorage->set('c', ['name' => 'c']);
+    $this->assertSame(['b', 'c'], array_keys($this->projectStorage->getAll()));
+
+    // Add project 'a' and confirm 'a' is first.
+    $this->projectStorage->set('a', ['name' => 'a']);
+    $this->assertSame(['a', 'b', 'c'], array_keys($this->projectStorage->getAll()));
+
+    // Add project 'd' with a negative weight and confirm 'd' is first.
+    $this->projectStorage->set('d', ['name' => 'd', 'weight' => -1]);
+    $this->assertSame(['d', 'a', 'b', 'c'], array_keys($this->projectStorage->getAll()));
+
+    // Add project 'aa' with a positive weight and confirm 'aa' is last.
+    $this->projectStorage->set('aa', ['name' => 'aa', 'weight' => 1]);
+    $this->assertSame(['d', 'a', 'b', 'c', 'aa'], array_keys($this->projectStorage->getAll()));
+
+    // Delete project 'a'.
+    $this->projectStorage->delete('a');
+    $this->assertSame(['d', 'b', 'c', 'aa'], array_keys($this->projectStorage->getAll()));
+
+    // Add project 'e' with a lower negative weight than 'd' and confirm 'e' is
+    // first.
+    $this->projectStorage->set('e', ['name' => 'e', 'weight' => -5]);
+    $this->assertSame(['e', 'd', 'b', 'c', 'aa'], array_keys($this->projectStorage->getAll()));
+
+    // Pretend there is a container rebuild by generating a new
+    // LocaleProjectStorage object with the same data.
+    $this->projectStorage = new LocaleProjectStorage($this->keyValueMemoryFactory);
+    $this->projectStorage->set('z', ['name' => 'z']);
+    $this->assertSame(['e', 'd', 'b', 'c', 'z', 'aa'], array_keys($this->projectStorage->getAll()));
+
+    // Now delete all projects.
+    $this->projectStorage->deleteAll();
+    $this->assertSame([], $this->projectStorage->getAll());
+
+    // Add project 'z' before project 'a' and confirm 'a' is first.
+    $this->projectStorage->set('z', ['name' => 'z']);
+    $this->projectStorage->set('a', ['name' => 'a']);
+    $this->assertSame(['a', 'z'], array_keys($this->projectStorage->getAll()));
+  }
+
+  /**
+   * Tests deleted projects are not included in the count.
+   */
+  public function testDelete(): void {
+    $this->projectStorage->set('b', ['name' => 'b']);
+    $this->assertSame(['name' => 'b'], $this->projectStorage->get('b'));
+    $this->assertSame(1, $this->projectStorage->countProjects());
+    $this->projectStorage->delete('b');
+    $this->assertNull($this->projectStorage->get('b'));
+    $this->assertSame(0, $this->projectStorage->countProjects());
+  }
+
+}

core/modules/locale/tests/src/Unit/LocaleTest.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit;
+
+use Drupal\Core\DependencyInjection\Container;
+use Drupal\locale\Locale;
+use Drupal\locale\LocaleConfigManager;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\locale\Locale
+ * @group Cache
+ */
+class LocaleTest extends UnitTestCase {
+
+  /**
+   * Tests deprecation of config() method.
+   *
+   * @covers ::config
+   * @group legacy
+   */
+  public function testConfig(): void {
+    $config_manager = $this->prophesize(LocaleConfigManager::class);
+    $container = $this->prophesize(Container::class);
+    $container->get('locale.config_manager')
+      ->willReturn($config_manager->reveal());
+    \Drupal::setContainer($container->reveal());
+
+    $this->expectDeprecation('The Drupal\locale\Locale is deprecated in drupal:10.3.0 and is removed from drupal:11.0.0. There is no replacement. See https://www.drupal.org/node/3437110');
+    $this->expectDeprecation('Drupal\locale\Locale::config() is deprecated in drupal:10.3.0 and is removed from drupal:11.0.0. Use \Drupal::service(\'locale.config_manager\') instead. See https://www.drupal.org/node/3437110');
+
+    $this->assertInstanceOf(LocaleConfigManager::class, Locale::config());
+  }
+
+}

core/modules/locale/tests/src/Unit/LocaleTranslationTest.php

@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Lock\LockBackendInterface;
+use Drupal\locale\LocaleTranslation;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * @coversDefaultClass \Drupal\locale\LocaleTranslation
+ * @group locale
+ */
+class LocaleTranslationTest extends UnitTestCase {
+
+  /**
+   * A mocked storage to use when instantiating LocaleTranslation objects.
+   *
+   * @var \PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $storage;
+
+  /**
+   * A mocked lock to use when instantiating LocaleTranslation objects.
+   *
+   * @var \Drupal\Core\Lock\LockBackendInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected LockBackendInterface $lock;
+
+  /**
+   * A mocked cache to use when instantiating LocaleTranslation objects.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected CacheBackendInterface $cache;
+
+  /**
+   * A mocked language manager built from LanguageManagerInterface.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $languageManager;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->storage = $this->createMock('Drupal\locale\StringStorageInterface');
+    $this->cache = $this->createMock('Drupal\Core\Cache\CacheBackendInterface');
+    $this->lock = $this->createMock('Drupal\Core\Lock\LockBackendInterface');
+    $this->languageManager = $this->createMock('Drupal\Core\Language\LanguageManagerInterface');
+    $this->requestStack = new RequestStack();
+  }
+
+  /**
+   * Tests for \Drupal\locale\LocaleTranslation::destruct().
+   */
+  public function testDestruct(): void {
+    $translation = new LocaleTranslation($this->storage, $this->cache, $this->lock, $this->getConfigFactoryStub(), $this->languageManager, $this->requestStack);
+    // Prove that destruction works without errors when translations are empty.
+    $this->assertNull($translation->destruct());
+  }
+
+}

core/modules/locale/tests/src/Unit/Menu/LocaleLocalTasksTest.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit\Menu;
+
+use Drupal\Tests\Core\Menu\LocalTaskIntegrationTestBase;
+
+/**
+ * Tests locale local tasks.
+ *
+ * @group locale
+ */
+class LocaleLocalTasksTest extends LocalTaskIntegrationTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    $this->directoryList = [
+      'locale' => 'core/modules/locale',
+    ];
+    parent::setUp();
+  }
+
+  /**
+   * Checks locale listing local tasks.
+   *
+   * @dataProvider getLocalePageRoutes
+   */
+  public function testLocalePageLocalTasks($route): void {
+    $tasks = [
+      0 => ['locale.translate_page', 'locale.translate_import', 'locale.translate_export', 'locale.settings'],
+    ];
+    $this->assertLocalTasks($route, $tasks);
+  }
+
+  /**
+   * Provides a list of routes to test.
+   */
+  public static function getLocalePageRoutes() {
+    return [
+      ['locale.translate_page'],
+      ['locale.translate_import'],
+      ['locale.translate_export'],
+      ['locale.settings'],
+    ];
+  }
+
+}

core/modules/locale/tests/src/Unit/StringBaseTest.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\locale\Unit;
+
+use Drupal\locale\SourceString;
+use Drupal\locale\StringStorageException;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\locale\StringBase
+ * @group locale
+ */
+class StringBaseTest extends UnitTestCase {
+
+  /**
+   * @covers ::save
+   */
+  public function testSaveWithoutStorage(): void {
+    $string = new SourceString(['source' => 'test']);
+    $this->expectException(StringStorageException::class);
+    $this->expectExceptionMessage('The string cannot be saved because its not bound to a storage: test');
+    $string->save();
+  }
+
+  /**
+   * @covers ::delete
+   */
+  public function testDeleteWithoutStorage(): void {
+    $string = new SourceString(['lid' => 1, 'source' => 'test']);
+    $this->expectException(StringStorageException::class);
+    $this->expectExceptionMessage('The string cannot be deleted because its not bound to a storage: test');
+    $string->delete();
+  }
+
+}

core/modules/locale/tests/test.af.po

@@ -0,0 +1,13 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "@site is currently under maintenance. We should be back shortly. Thank you for your patience."
+msgstr "Ons is tans besig met onderhoud op @site. Wees asseblief geduldig, ons sal binnekort weer terug wees."
+
+msgid "Locale can translate"
+msgstr "Locale can translate Afrikaans"

core/modules/locale/tests/test.de.po

@@ -0,0 +1,28 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 7\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Monday"
+msgstr "Montag"
+
+msgid "Tuesday"
+msgstr "Dienstag"
+
+msgid "Wednesday"
+msgstr "Mittwoch"
+
+msgid "Thursday"
+msgstr "Donnerstag"
+
+msgid "Friday"
+msgstr "Freitag"
+
+msgid "Saturday"
+msgstr "Samstag"
+
+msgid "Sunday"
+msgstr "Sonntag"

core/modules/locale/tests/test.nl.po

@@ -0,0 +1,31 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 7\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Monday"
+msgstr "maandag"
+
+msgid "Tuesday"
+msgstr "dinsdag"
+
+msgid "Wednesday"
+msgstr "woensdag"
+
+msgid "Thursday"
+msgstr "donderdag"
+
+msgid "Extra day"
+msgstr "extra dag"
+
+msgid "Friday"
+msgstr "vrijdag"
+
+msgid "Saturday"
+msgstr "zaterdag"
+
+msgid "Sunday"
+msgstr "zondag"

core/modules/locale/tests/test.xx.po

@@ -0,0 +1,40 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 7\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "Monday"
+msgstr "lundi"
+
+msgid "Tuesday"
+msgstr "mardi"
+
+msgid "Wednesday"
+msgstr "mercredi"
+
+msgid "Thursday"
+msgstr "jeudi"
+
+msgid "Friday"
+msgstr "vendredi"
+
+msgid "Saturday"
+msgstr "samedi"
+
+msgid "Sunday"
+msgstr "dimanche"
+
+msgid "Allowed HTML source string"
+msgstr "<strong>Allowed HTML translation string</strong>"
+
+msgid "Another allowed HTML source string"
+msgstr "<script>Disallowed HTML translation string</script>"
+
+msgid "Source string for multiline translation"
+msgstr ""
+"Multiline translation string "
+"to make sure that "
+"import works with it."