🐛(frontend) switch language when the current one is untranslated

With zh-tw active, selecting English did nothing: its bundle is empty, so
i18next reports `resolvedLanguage` as en while `language` is zh-tw, and
the hook skipped the change. Compare with `language` instead, and add
unit tests on the language pickers.

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to
 ### Fixed
 
 - 🐛(backend) bypass throttling on access/me action from yhub
+- 🐛(frontend) switch language when the current one is untranslated #2790
 
 ## [v6.0.0] - 2026-10-05
 

src/frontend/apps/impress/src/features/language/components/__tests__/LanguagePicker.spec.tsx

@@ -0,0 +1,105 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import i18next from 'i18next';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { AppWrapper } from '@/tests/utils';
+
+import { LanguagePicker, LanguagePickerLegacy } from '../LanguagePicker';
+
+const LANGUAGES = [
+  ['en-us', 'English'],
+  ['fr-fr', 'Français'],
+  ['zh-cn', '中文'],
+  ['zh-tw', '繁體中文'],
+];
+
+const updateUser = vi.fn();
+
+vi.mock('@/core', async () => {
+  const actual = await vi.importActual('@/core');
+
+  return {
+    ...actual,
+    useConfig: () => ({ data: { LANGUAGES } }),
+  };
+});
+
+vi.mock('@/core/config/api/useConfig', () => ({
+  useConfig: () => ({ data: { LANGUAGES } }),
+}));
+
+vi.mock('@/core/api/useUserUpdate', () => ({
+  useUserUpdate: () => ({ mutateAsync: updateUser }),
+}));
+
+vi.mock('@/features/auth', async () => {
+  const actual = await vi.importActual('@/features/auth');
+
+  return {
+    ...actual,
+    useAuthQuery: () => ({
+      data: { id: 'user-1', language: 'zh-tw' },
+    }),
+  };
+});
+
+const kitSelection = () =>
+  Array.from(document.querySelectorAll('.c__dropdown-menu-item__check')).map(
+    (check) => check.closest('[role="menuitem"]')?.textContent,
+  );
+
+const legacySelection = () =>
+  screen
+    .getAllByRole('menuitemradio')
+    .filter((item) => item.getAttribute('aria-checked') === 'true')
+    .map((item) => item.textContent?.replace('check', ''));
+
+describe.each([
+  ['LanguagePicker', LanguagePicker, 'menuitem', kitSelection],
+  [
+    'LanguagePickerLegacy',
+    LanguagePickerLegacy,
+    'menuitemradio',
+    legacySelection,
+  ],
+] as const)('<%s />', (_name, Picker, itemRole, getSelection) => {
+  beforeEach(async () => {
+    updateUser.mockReset();
+    updateUser.mockResolvedValue({});
+    await i18next.changeLanguage('zh-tw');
+  });
+
+  afterEach(async () => {
+    await i18next.changeLanguage('en');
+  });
+
+  const openPicker = async () => {
+    render(<Picker />, { wrapper: AppWrapper });
+    await userEvent.click(screen.getByRole('button'));
+  };
+
+  it('marks a single language as selected', async () => {
+    await openPicker();
+
+    expect(getSelection()).toEqual(['繁體中文']);
+  });
+
+  it('switches from zh-tw to English and keeps it', async () => {
+    await openPicker();
+
+    await userEvent.click(screen.getByRole(itemRole, { name: 'English' }));
+
+    await waitFor(() => {
+      expect(i18next.language).toBe('en');
+    });
+    expect(updateUser).toHaveBeenCalledWith({
+      id: 'user-1',
+      language: 'en-us',
+    });
+
+    // The picker follows the language and still shows a single selection
+    await userEvent.click(screen.getByRole('button'));
+    expect(getSelection()).toEqual(['English']);
+  });
+});

src/frontend/apps/impress/src/features/language/hooks/useSynchronizedLanguage.ts

@@ -41,10 +41,7 @@ export const useSynchronizedLanguage = () => {
         availableFrontendLanguages,
         [language],
       )[0];
-      if (
-        i18n.isInitialized &&
-        i18n.resolvedLanguage !== closestFrontendLanguage
-      ) {
+      if (i18n.isInitialized && i18n.language !== closestFrontendLanguage) {
         await i18n.changeLanguage(closestFrontendLanguage);
       }
     },