diff --git a/CHANGELOG.md b/CHANGELOG.md
index d6c19de42..a421f9514 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,20 @@ ones are marked like "v1.0.0-fork".
 
 ## [Unreleased]
 
+### Fixed
+
+* **The interface language setting had no effect on multi-user installs.** The
+  language was read before the signed-in user was known, so everyone got the
+  default. It now applies to every signed-in page, and the preferences page
+  reloads to show it as soon as it is saved.
+
+* **The language picker fell out of step with the page.** On multi-user installs
+  `?filterlang=` links never changed the current language, and creating a
+  language, opening a text or reviewing a language left the picker on the old
+  one. All of these now update it. Remembered sort orders had the same fault
+  and now persist per user, and none of these values reach the shared defaults
+  any more.
+
 ## [3.8.1-fork] - 2026-11-04
 
 ### Changed in 3.8.1-fork
diff --git a/cypress.config.ts b/cypress.config.ts
index e99f9392b..7544ff858 100644
--- a/cypress.config.ts
+++ b/cypress.config.ts
@@ -3,6 +3,21 @@ import { defineConfig } from 'cypress';
 export default defineConfig({
   e2e: {
     baseUrl: 'http://localhost:8000',
+    setupNodeEvents(on) {
+      on('task', {
+        // In Node, because cy.request() fails the test on a refused connection.
+        ankiConnectVersion(url: string): Promise<number | null> {
+          return fetch(url, {
+            method: 'POST',
+            body: JSON.stringify({ action: 'version', version: 6 }),
+            signal: AbortSignal.timeout(3000)
+          })
+            .then((response) => response.json())
+            .then((body: { result?: number }) => body.result ?? null)
+            .catch(() => null);
+        }
+      });
+    },
     supportFile: 'cypress/support/e2e.ts',
     specPattern: 'cypress/e2e/**/*.cy.ts',
     viewportWidth: 1280,
diff --git a/cypress/e2e/24-anki-live.cy.ts b/cypress/e2e/24-anki-live.cy.ts
index 8c4d0590d..e8948d109 100644
--- a/cypress/e2e/24-anki-live.cy.ts
+++ b/cypress/e2e/24-anki-live.cy.ts
@@ -51,12 +51,9 @@ describe('Sync reviews with a real Anki', () => {
   const extraDecks: string[] = [];
 
   before(() => {
-    cy.request({ method: 'POST', url: anki, body: { action: 'version', version: 6 }, failOnStatusCode: false, timeout: 3000 })
-      .then((response) => {
-        ankiUp = response.status === 200 && (response.body as { result?: number }).result === 6;
-      }, () => {
-        ankiUp = false;
-      });
+    cy.task<number | null>('ankiConnectVersion', anki).then((version) => {
+      ankiUp = version === 6;
+    });
   });
 
   beforeEach(function () {
diff --git a/docs-src/changelog.md b/docs-src/changelog.md
index 8733f25cf..faadda95b 100644
--- a/docs-src/changelog.md
+++ b/docs-src/changelog.md
@@ -12,6 +12,20 @@ ones are marked like "v1.0.0-fork".
 
 ## [Unreleased]
 
+### Fixed
+
+* **The interface language setting had no effect on multi-user installs.** The
+  language was read before the signed-in user was known, so everyone got the
+  default. It now applies to every signed-in page, and the preferences page
+  reloads to show it as soon as it is saved.
+
+* **The language picker fell out of step with the page.** On multi-user installs
+  `?filterlang=` links never changed the current language, and creating a
+  language, opening a text or reviewing a language left the picker on the old
+  one. All of these now update it. Remembered sort orders had the same fault
+  and now persist per user, and none of these values reach the shared defaults
+  any more.
+
 ## [3.8.1-fork] - 2026-11-04
 
 ### Changed in 3.8.1-fork
diff --git a/src/Modules/Language/Http/LanguageApiHandler.php b/src/Modules/Language/Http/LanguageApiHandler.php
index 678a4ea27..692571657 100644
--- a/src/Modules/Language/Http/LanguageApiHandler.php
+++ b/src/Modules/Language/Http/LanguageApiHandler.php
@@ -26,6 +26,7 @@
 use Lwt\Shared\Infrastructure\Http\JsonResponse;
 use Lwt\Api\V1\Response;
 use Lwt\Modules\Language\Application\LanguageFacade;
+use Lwt\Shared\Infrastructure\Language\CurrentLanguage;
 use Lwt\Shared\Infrastructure\Language\LanguagePresets;
 use Lwt\Modules\Text\Application\Services\SentenceService;
 use Lwt\Modules\Vocabulary\Application\UseCases\FindSimilarTerms;
@@ -368,6 +369,7 @@ public function formatCreate(array $data): array
 
         $id = $this->languageFacade->createFromData($data);
         if ($id > 0) {
+            CurrentLanguage::follow($id);
             return ['success' => true, 'id' => $id];
         }
 
@@ -501,7 +503,7 @@ public function formatGetDefinitions(): array
      */
     public function formatSetDefault(int $id): array
     {
-        Settings::save('currentlanguage', (string)$id);
+        Settings::savePerUser('currentlanguage', (string)$id);
         return ['success' => true];
     }
 
diff --git a/src/Modules/Review/Http/ReviewController.php b/src/Modules/Review/Http/ReviewController.php
index 0c7a2ca26..f4eecbb05 100644
--- a/src/Modules/Review/Http/ReviewController.php
+++ b/src/Modules/Review/Http/ReviewController.php
@@ -23,6 +23,7 @@
 use Lwt\Modules\Review\Application\ReviewFacade;
 use Lwt\Modules\Review\Infrastructure\SessionStateManager;
 use Lwt\Modules\Language\Application\LanguageFacade;
+use Lwt\Shared\Infrastructure\Language\CurrentLanguage;
 use Lwt\Shared\Infrastructure\Language\LanguagePresets;
 use Lwt\Shared\UI\Helpers\PageLayoutHelper;
 
@@ -177,6 +178,7 @@ private function renderReviewPage(): void
         // Get language settings
         $langIdFromSql = $this->reviewFacade->getLanguageIdFromReviewSql($reviewsql, $reviewParams);
         if ($langIdFromSql === null) {
+            CurrentLanguage::follow($langId ?? 0);
             PageLayoutHelper::renderPageStartNobody(__('review.page_title'), 'full-width');
             include __DIR__ . '/../Views/no_terms.php';
             PageLayoutHelper::renderPageEnd();
@@ -235,6 +237,7 @@ private function renderReviewPage(): void
             'property' => $testData['property']
         ];
 
+        CurrentLanguage::follow($langIdFromSql);
         PageLayoutHelper::renderPageStartNobody(__('review.page_title'), 'full-width');
         include __DIR__ . '/../Views/review_desktop.php';
         PageLayoutHelper::renderPageEnd();
diff --git a/src/Modules/Text/Http/TextReadController.php b/src/Modules/Text/Http/TextReadController.php
index 8a9c38415..7e0cb596a 100644
--- a/src/Modules/Text/Http/TextReadController.php
+++ b/src/Modules/Text/Http/TextReadController.php
@@ -22,6 +22,7 @@
 use Lwt\Modules\Text\Application\Services\TextDisplayService;
 use Lwt\Modules\Text\Application\Services\TextNavigationService;
 use Lwt\Shared\Infrastructure\Database\Settings;
+use Lwt\Shared\Infrastructure\Language\CurrentLanguage;
 use Lwt\Shared\UI\Helpers\PageLayoutHelper;
 use Lwt\Shared\Infrastructure\Http\RedirectResponse;
 use Lwt\Modules\Activity\Infrastructure\MySqlActivityRepository;
@@ -148,6 +149,7 @@ public function renderReadPage(int $textId): ?RedirectResponse
         // neither is rendered server-side here.
 
         Settings::savePerUser('currenttext', $textId);
+        CurrentLanguage::follow($langId);
 
         PageLayoutHelper::renderPageStartNobody(__('text.titles.read'), 'full-width');
         include self::MODULE_VIEWS . '/read_desktop.php';
diff --git a/src/Shared/I18n/UserLocale.php b/src/Shared/I18n/UserLocale.php
new file mode 100644
index 000000000..3771b78a8
--- /dev/null
+++ b/src/Shared/I18n/UserLocale.php
@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * Applies the signed-in user's interface language.
+ *
+ * PHP version 8.2
+ *
+ * @category Lwt
+ * @package  Lwt\Shared\I18n
+ * @author   HugoFara <git@hugofara.net>
+ * @license  Unlicense <http://unlicense.org/>
+ * @link     https://hugofara.github.io/lwt/developer/api
+ * @since    3.8.2
+ */
+
+declare(strict_types=1);
+
+namespace Lwt\Shared\I18n;
+
+use Lwt\Shared\Infrastructure\Container\Container;
+use Lwt\Shared\Infrastructure\Database\Settings;
+
+/**
+ * Sets the translator to the language the signed-in user chose.
+ *
+ * TranslatorServiceProvider::boot() runs before the middleware restores the
+ * user, so on a multi-user install it can only see the global default. The
+ * middleware calls this once the user is known. The user's own choice also
+ * beats a leftover `lwt_lang` cookie, which only exists to serve guests.
+ *
+ * A user who never chose keeps whatever boot() settled on, so a language picked
+ * on the sign-up page lasts until they pick one in their preferences.
+ */
+final class UserLocale
+{
+    /**
+     * Apply the current user's stored interface language, if they have one.
+     *
+     * @return void
+     */
+    public static function apply(): void
+    {
+        $container = Container::getInstance();
+        if (!$container->has(Translator::class)) {
+            return;
+        }
+
+        try {
+            $locale = Settings::getOwnValue('app_language');
+        } catch (\RuntimeException $e) {
+            return;
+        }
+
+        $translator = $container->getTyped(Translator::class);
+        if ($locale !== '' && in_array($locale, $translator->getAvailableLocales(), true)) {
+            $translator->setLocale($locale);
+        }
+    }
+}
diff --git a/src/Shared/Infrastructure/Database/Settings.php b/src/Shared/Infrastructure/Database/Settings.php
index 8b6a5f263..84b265c28 100644
--- a/src/Shared/Infrastructure/Database/Settings.php
+++ b/src/Shared/Infrastructure/Database/Settings.php
@@ -140,6 +140,30 @@ public static function getWithDefault(string $key): string
         return SettingDefinitions::getDefault($key) ?? '';
     }
 
+    /**
+     * Get the value the current user saved themselves.
+     *
+     * Unlike getWithDefault(), this tells "chose the default" from "never
+     * chose", which matters when a choice has to beat another source.
+     *
+     * @param string $key Setting key
+     *
+     * @return string The user's own value, '' if they have none or nobody is signed in
+     */
+    public static function getOwnValue(string $key): string
+    {
+        $userId = Globals::getCurrentUserId();
+        if ($key === '' || $userId === null) {
+            return '';
+        }
+
+        return trim((string) Connection::preparedFetchValue(
+            "SELECT StValue FROM settings WHERE StKey = ? AND StUsID = ?",
+            [$key, $userId],
+            'StValue'
+        ));
+    }
+
     /**
      * Save the setting identified by a key with a specific value.
      *
diff --git a/src/Shared/Infrastructure/Http/InputValidator.php b/src/Shared/Infrastructure/Http/InputValidator.php
index e45c770ec..05b4a3cee 100644
--- a/src/Shared/Infrastructure/Http/InputValidator.php
+++ b/src/Shared/Infrastructure/Http/InputValidator.php
@@ -874,7 +874,7 @@ public static function getStringWithDb(
 
         if (self::has($reqKey)) {
             $value = self::getString($reqKey);
-            \Lwt\Shared\Infrastructure\Database\Settings::save($dbKey, $value);
+            \Lwt\Shared\Infrastructure\Database\Settings::savePerUser($dbKey, $value);
             return $value;
         }
 
@@ -906,7 +906,7 @@ public static function getIntWithDb(
 
         if (self::has($reqKey)) {
             $value = self::getInt($reqKey, $default);
-            \Lwt\Shared\Infrastructure\Database\Settings::save($dbKey, (string) ($value ?? $default));
+            \Lwt\Shared\Infrastructure\Database\Settings::savePerUser($dbKey, (string) ($value ?? $default));
             return $value ?? $default;
         }
 
diff --git a/src/Shared/Infrastructure/Language/CurrentLanguage.php b/src/Shared/Infrastructure/Language/CurrentLanguage.php
index 55dc28021..c9fd3dcea 100644
--- a/src/Shared/Infrastructure/Language/CurrentLanguage.php
+++ b/src/Shared/Infrastructure/Language/CurrentLanguage.php
@@ -23,10 +23,10 @@
 /**
  * Resolves the language the user is currently working in.
  *
- * The `currentlanguage` setting is only written when the user actively picks a
- * language from the navbar dropdown. Two situations leave it unset or stale, and
- * both used to surface as "no language selected" even though the navbar showed
- * one:
+ * The `currentlanguage` setting is written when the user picks a language from
+ * the navbar dropdown, or when they start working in one (see follow()). Two
+ * situations can still leave it unset or stale, and both used to surface as
+ * "no language selected" even though the navbar showed one:
  *
  * - A user who has just created their first language has never fired the
  *   dropdown's `change` event. With exactly one language there is no other
@@ -39,8 +39,8 @@
  * browser falls back to displaying the first option. The display and the stored
  * state disagreed, and every server-side consumer read 0.
  *
- * This resolver is the single answer to "which language is active?". It is
- * read-only on purpose: it never writes the setting, so a plain GET has no side
+ * resolveId() is the single answer to "which language is active?". It is
+ * read-only on purpose: it never writes the setting, so resolving has no side
  * effects and the same request always resolves the same way.
  *
  * Callers that need the id to agree with what the navbar displays must use this
@@ -72,6 +72,27 @@ public static function resolveId(): int
         }
     }
 
+    /**
+     * Make a language current because the user started working in it.
+     *
+     * Keeps the navbar in step with the page: without this, opening a text in
+     * another language leaves the navbar and the next filtered page on the old
+     * one. Only the user's own languages count, since the lookup is user-scoped.
+     *
+     * @param int $languageId Language the user is now working in
+     *
+     * @return void
+     */
+    public static function follow(int $languageId): void
+    {
+        if ($languageId <= 0 || (int) Settings::get('currentlanguage') === $languageId) {
+            return;
+        }
+        if (self::exists($languageId)) {
+            Settings::savePerUser('currentlanguage', (string) $languageId);
+        }
+    }
+
     /**
      * Check whether a language ID still exists.
      *
diff --git a/src/Shared/Infrastructure/Routing/Middleware/AdminMiddleware.php b/src/Shared/Infrastructure/Routing/Middleware/AdminMiddleware.php
index 784f0f451..2e3ae0885 100644
--- a/src/Shared/Infrastructure/Routing/Middleware/AdminMiddleware.php
+++ b/src/Shared/Infrastructure/Routing/Middleware/AdminMiddleware.php
@@ -18,6 +18,7 @@
 namespace Lwt\Shared\Infrastructure\Routing\Middleware;
 
 use Lwt\Modules\User\Domain\User;
+use Lwt\Shared\I18n\UserLocale;
 use Lwt\Shared\Infrastructure\Globals;
 use Lwt\Modules\User\Application\UserFacade;
 use Lwt\Shared\Infrastructure\Container\Container;
@@ -90,6 +91,7 @@ public function handle(): bool
             return false;
         }
 
+        UserLocale::apply();
         return true;
     }
 
diff --git a/src/Shared/Infrastructure/Routing/Middleware/AuthMiddleware.php b/src/Shared/Infrastructure/Routing/Middleware/AuthMiddleware.php
index 722e32e08..01d32ef0b 100644
--- a/src/Shared/Infrastructure/Routing/Middleware/AuthMiddleware.php
+++ b/src/Shared/Infrastructure/Routing/Middleware/AuthMiddleware.php
@@ -17,6 +17,7 @@
 
 namespace Lwt\Shared\Infrastructure\Routing\Middleware;
 
+use Lwt\Shared\I18n\UserLocale;
 use Lwt\Shared\Infrastructure\Globals;
 use Lwt\Modules\User\Application\UserFacade;
 use Lwt\Shared\Infrastructure\Container\Container;
@@ -84,11 +85,13 @@ public function handle(): bool
 
         // Try session authentication
         if ($this->validateSession()) {
+            UserLocale::apply();
             return true;
         }
 
         // Try API token authentication
         if ($this->validateApiToken()) {
+            UserLocale::apply();
             return true;
         }
 
diff --git a/src/frontend/js/modules/user/pages/preferences_app.ts b/src/frontend/js/modules/user/pages/preferences_app.ts
index 73ff857b6..9c872d8b9 100644
--- a/src/frontend/js/modules/user/pages/preferences_app.ts
+++ b/src/frontend/js/modules/user/pages/preferences_app.ts
@@ -19,6 +19,7 @@ import { ProfileApi } from '../api/profile_api';
 /** Alpine state for the preferences page. */
 export interface PreferencesAppData {
   settings: Record<string, string>;
+  loadedLocale: string | undefined;
   isLoading: boolean;
   isSaving: boolean;
   error: string;
@@ -41,6 +42,7 @@ export interface PreferencesAppData {
 export function preferencesAppData(): PreferencesAppData {
   return {
     settings: {},
+    loadedLocale: undefined,
     isLoading: false,
     isSaving: false,
     error: '',
@@ -63,6 +65,7 @@ export function preferencesAppData(): PreferencesAppData {
       }
 
       this.settings = response.data.settings;
+      this.loadedLocale = this.settings.app_language;
     },
 
     async save(): Promise<void> {
@@ -81,6 +84,12 @@ export function preferencesAppData(): PreferencesAppData {
         return;
       }
 
+      if (this.settings.app_language !== this.loadedLocale) {
+        // The page is already rendered in the old language.
+        window.location.reload();
+        return;
+      }
+
       this.success = t('user.flash.preferences_saved');
     },
 
diff --git a/tests/backend/Shared/Integration/MultiUserCurrentLanguageIntegrationTest.php b/tests/backend/Shared/Integration/MultiUserCurrentLanguageIntegrationTest.php
new file mode 100644
index 000000000..86c12902d
--- /dev/null
+++ b/tests/backend/Shared/Integration/MultiUserCurrentLanguageIntegrationTest.php
@@ -0,0 +1,244 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Lwt\Tests\Shared\Integration;
+
+use Lwt\Modules\Language\Http\LanguageApiHandler;
+use Lwt\Shared\I18n\Translator;
+use Lwt\Shared\I18n\UserLocale;
+use Lwt\Shared\Infrastructure\Bootstrap\EnvLoader;
+use Lwt\Shared\Infrastructure\Container\Container;
+use Lwt\Shared\Infrastructure\Database\Configuration;
+use Lwt\Shared\Infrastructure\Database\Connection;
+use Lwt\Shared\Infrastructure\Database\Settings;
+use Lwt\Shared\Infrastructure\Globals;
+use Lwt\Shared\Infrastructure\Http\InputValidator;
+use Lwt\Shared\Infrastructure\Language\CurrentLanguage;
+use PHPUnit\Framework\Attributes\Group;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * The current language and interface language of a signed-in user.
+ *
+ * Production runs in multi-user mode and the suite mostly does not. There,
+ * values saved to the global row never reach the reader, and the translator
+ * boots before the user is known, so both of these silently did nothing.
+ */
+#[Group('integration')]
+class MultiUserCurrentLanguageIntegrationTest extends TestCase
+{
+    private const USERNAME = 'multi_user_current_language_test';
+    private const OTHER_USERNAME = 'multi_user_current_language_other';
+
+    private static bool $dbConnected = false;
+    private static int $userId = 0;
+    private static int $otherUserId = 0;
+    private static int $languageId = 0;
+    private static int $otherLanguageId = 0;
+    private static int $foreignLanguageId = 0;
+
+    public static function setUpBeforeClass(): void
+    {
+        $config = EnvLoader::getDatabaseConfig();
+        if (!Globals::getDbConnection()) {
+            try {
+                Globals::setDbConnection(Configuration::connect(
+                    $config['server'],
+                    $config['userid'],
+                    $config['passwd'],
+                    'test_' . $config['dbname'],
+                    $config['socket'] ?? ''
+                ));
+            } catch (\Throwable) {
+                return;
+            }
+        }
+        self::$dbConnected = true;
+
+        self::$userId = self::insertUser(self::USERNAME);
+        self::$otherUserId = self::insertUser(self::OTHER_USERNAME);
+        self::$languageId = self::insertLanguage('CurrentFirst', self::$userId);
+        self::$otherLanguageId = self::insertLanguage('CurrentSecond', self::$userId);
+        self::$foreignLanguageId = self::insertLanguage('CurrentForeign', self::$otherUserId);
+    }
+
+    public static function tearDownAfterClass(): void
+    {
+        foreach ([self::$userId, self::$otherUserId] as $userId) {
+            if ($userId !== 0) {
+                Connection::preparedExecute('DELETE FROM settings WHERE StUsID = ?', [$userId]);
+                Connection::preparedExecute('DELETE FROM users WHERE UsID = ?', [$userId]);
+            }
+        }
+    }
+
+    private static function insertUser(string $name): int
+    {
+        Connection::preparedExecute('DELETE FROM users WHERE UsUsername = ?', [$name]);
+        return (int) Connection::preparedInsert(
+            "INSERT INTO users (UsUsername, UsPasswordHash) VALUES (?, 'x')",
+            [$name]
+        );
+    }
+
+    private static function insertLanguage(string $name, int $userId): int
+    {
+        return (int) Connection::preparedInsert(
+            "INSERT INTO languages (
+                LgName, LgDict1URI, LgDict2URI, LgGoogleTranslateURI,
+                LgTextSize, LgRegexpSplitSentences, LgRegexpWordCharacters,
+                LgRemoveSpaces, LgSplitEachChar, LgRightToLeft, LgShowRomanization,
+                LgUsID
+            ) VALUES (?, '', '', '', 100, '.!?', 'a-zA-Z', 0, 0, 0, 0, ?)",
+            [$name, $userId]
+        );
+    }
+
+    protected function setUp(): void
+    {
+        if (!self::$dbConnected) {
+            $this->markTestSkipped('Database connection required');
+        }
+        Globals::setMultiUserEnabled(true);
+        Globals::setCurrentUserId(self::$userId);
+        Connection::preparedExecute(
+            'DELETE FROM settings WHERE StUsID = ?',
+            [self::$userId]
+        );
+        Settings::savePerUser('currentlanguage', (string) self::$languageId);
+    }
+
+    protected function tearDown(): void
+    {
+        unset($_GET['filterlang'], $_REQUEST['filterlang'], $_GET['sort'], $_REQUEST['sort']);
+        Globals::setCurrentUserId(null);
+        Globals::setMultiUserEnabled(false);
+        Container::setInstance(null);
+    }
+
+    private function globalValue(string $key): ?string
+    {
+        $value = Connection::preparedFetchValue(
+            'SELECT StValue FROM settings WHERE StKey = ? AND StUsID = 0',
+            [$key],
+            'StValue'
+        );
+        return $value === null ? null : (string) $value;
+    }
+
+    private function translatorIn(string $locale): Translator
+    {
+        $translator = new Translator(getcwd() . '/locale', 'en');
+        $translator->setLocale($locale);
+        $container = new Container();
+        $container->instance(Translator::class, $translator);
+        Container::setInstance($container);
+        return $translator;
+    }
+
+    public function testFilterParameterReachesTheNavbarAndNotTheGlobalRow(): void
+    {
+        $globalBefore = $this->globalValue('currentlanguage');
+        $_GET['filterlang'] = $_REQUEST['filterlang'] = (string) self::$otherLanguageId;
+
+        InputValidator::getStringWithDb('filterlang', 'currentlanguage');
+
+        $this->assertSame(self::$otherLanguageId, CurrentLanguage::resolveId());
+        $this->assertSame($globalBefore, $this->globalValue('currentlanguage'));
+    }
+
+    public function testRememberedSortOrderIsPerUser(): void
+    {
+        $globalBefore = $this->globalValue('currentwordsort');
+        $_GET['sort'] = $_REQUEST['sort'] = '3';
+
+        InputValidator::getIntWithDb('sort', 'currentwordsort', 1);
+
+        $this->assertSame('3', Settings::get('currentwordsort'));
+        $this->assertSame($globalBefore, $this->globalValue('currentwordsort'));
+    }
+
+    public function testFollowingALanguageMakesItCurrent(): void
+    {
+        CurrentLanguage::follow(self::$otherLanguageId);
+
+        $this->assertSame(self::$otherLanguageId, CurrentLanguage::resolveId());
+    }
+
+    public function testAnotherUsersLanguageCannotBecomeCurrent(): void
+    {
+        CurrentLanguage::follow(self::$foreignLanguageId);
+
+        // The stored row, since resolveId() would hide a bad write behind its fallback.
+        $this->assertSame((string) self::$languageId, Settings::getOwnValue('currentlanguage'));
+    }
+
+    public function testCreatingALanguageMakesItCurrent(): void
+    {
+        $result = (new LanguageApiHandler())->formatCreate(['name' => 'CurrentCreated']);
+
+        $this->assertTrue($result['success']);
+        $this->assertSame($result['id'], CurrentLanguage::resolveId());
+    }
+
+    public function testSettingTheDefaultLanguageIsPerUser(): void
+    {
+        $globalBefore = $this->globalValue('currentlanguage');
+
+        (new LanguageApiHandler())->formatSetDefault(self::$otherLanguageId);
+
+        $this->assertSame(self::$otherLanguageId, CurrentLanguage::resolveId());
+        $this->assertSame($globalBefore, $this->globalValue('currentlanguage'));
+    }
+
+    public function testUsersOwnInterfaceLanguageIsApplied(): void
+    {
+        $translator = $this->translatorIn('en');
+        Settings::savePerUser('app_language', 'de');
+
+        UserLocale::apply();
+
+        $this->assertSame('de', $translator->getLocale());
+    }
+
+    public function testUsersChoiceBeatsWhatBootSettledOn(): void
+    {
+        // Boot applies the lwt_lang cookie, so this stands for a stale one.
+        $translator = $this->translatorIn('fr');
+        Settings::savePerUser('app_language', 'en');
+
+        UserLocale::apply();
+
+        $this->assertSame('en', $translator->getLocale());
+    }
+
+    public function testNoChoiceKeepsTheLocaleFromBoot(): void
+    {
+        $translator = $this->translatorIn('fr');
+
+        UserLocale::apply();
+
+        $this->assertSame('fr', $translator->getLocale());
+    }
+
+    public function testAnotherUsersChoiceIsNotApplied(): void
+    {
+        $translator = $this->translatorIn('en');
+        Settings::saveForUser('app_language', 'de', self::$otherUserId);
+
+        UserLocale::apply();
+
+        $this->assertSame('en', $translator->getLocale());
+    }
+
+    public function testUnknownLocaleIsIgnored(): void
+    {
+        $translator = $this->translatorIn('en');
+        Settings::savePerUser('app_language', 'zz');
+
+        UserLocale::apply();
+
+        $this->assertSame('en', $translator->getLocale());
+    }
+}
diff --git a/tests/frontend/user/preferences_app.test.ts b/tests/frontend/user/preferences_app.test.ts
new file mode 100644
index 000000000..1f6da98dc
--- /dev/null
+++ b/tests/frontend/user/preferences_app.test.ts
@@ -0,0 +1,65 @@
+/**
+ * Tests for preferences_app.ts - saving a new interface language
+ */
+import { describe, it, expect, afterEach, beforeEach, vi } from 'vitest';
+import { preferencesAppData } from '../../../src/frontend/js/modules/user/pages/preferences_app';
+import { ProfileApi } from '../../../src/frontend/js/modules/user/api/profile_api';
+
+describe('preferences_app.ts', () => {
+  const reload = vi.fn();
+
+  beforeEach(() => {
+    vi.stubGlobal('location', { ...window.location, reload });
+    vi.spyOn(ProfileApi, 'getPreferences').mockResolvedValue({
+      data: { settings: { app_language: 'en', 'set-tts': '0' } }
+    } as never);
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.restoreAllMocks();
+    reload.mockClear();
+  });
+
+  async function loaded(): Promise<ReturnType<typeof preferencesAppData>> {
+    const app = preferencesAppData();
+    await app.load();
+    return app;
+  }
+
+  function saveAnswers(data: unknown): void {
+    vi.spyOn(ProfileApi, 'savePreferences').mockResolvedValue(data as never);
+  }
+
+  it('reloads after a new interface language is saved', async () => {
+    saveAnswers({ data: { success: true } });
+    const app = await loaded();
+    app.setValue('app_language', 'de');
+
+    await app.save();
+
+    expect(reload).toHaveBeenCalledTimes(1);
+  });
+
+  it('stays on the page when the interface language is unchanged', async () => {
+    saveAnswers({ data: { success: true } });
+    const app = await loaded();
+    app.setValue('set-tts', '1');
+
+    await app.save();
+
+    expect(reload).not.toHaveBeenCalled();
+    expect(app.success).not.toBe('');
+  });
+
+  it('does not reload when the save fails', async () => {
+    saveAnswers({ error: 'nope' });
+    const app = await loaded();
+    app.setValue('app_language', 'de');
+
+    await app.save();
+
+    expect(reload).not.toHaveBeenCalled();
+    expect(app.error).toBe('nope');
+  });
+});