test(i18n): make the language registry's own check able to fail

frontend/src/i18n/__tests__/i18n.test.ts

@@ -71,7 +71,11 @@ describe('i18n configuration', () => {
   });
 
   it('uses each registered language for the document and i18next', async () => {
-    expect(SUPPORTED_LANGUAGES).toEqual(LANGUAGE_REGISTRY.map(({ code }) => code));
+    // Pinned as a literal, not derived from LANGUAGE_REGISTRY: comparing the
+    // list to the map that defines it passes however the map changes
+    // (Sprint Review 5611893458 on #2399). v2/__tests__/languageSurfaces.test.tsx
+    // carries the same pin for the three surfaces that read the registry.
+    expect(SUPPORTED_LANGUAGES).toEqual(['en', 'zh-CN']);
     for (const { code } of LANGUAGE_REGISTRY) {
       await act(async () => { await i18n.changeLanguage(code); });
       expect(i18n.resolvedLanguage).toBe(code);

frontend/src/v2/__tests__/V2SettingsPage.test.tsx

@@ -1,6 +1,6 @@
 import React from 'react';
-import { act, fireEvent, render, screen, waitFor } from '@testing-library/react';
-import i18n, { i18nReady } from '../../i18n';
+import { act, fireEvent, render, screen, waitFor, within } from '@testing-library/react';
+import i18n, { i18nReady, LANGUAGE_REGISTRY } from '../../i18n';
 import axios from '../../utils/axiosConfig';
 import { MemoryRouter } from 'react-router-dom';
 import { AuthContext } from '../../context/AuthContext';
@@ -90,6 +90,20 @@ describe('V2SettingsPage', () => {
     await act(async () => { await i18n.changeLanguage('en'); });
   });
 
+  // Pinned as a literal AND compared with the registry: the first assertion
+  // fails when the shipped languages change, the second when this surface
+  // stops reading LANGUAGE_REGISTRY and drifts from it.
+  test('builds the language radios from the registry, and only those', async () => {
+    await i18nReady;
+    (axios.get as jest.Mock).mockResolvedValue({ data: { hasToken: false } });
+    renderSettings();
+
+    const radios = within(screen.getByRole('radiogroup', { name: 'Language' })).getAllByRole('radio');
+    const labels = radios.map((radio) => radio.textContent);
+    expect(labels).toEqual(['English', '中文']);
+    expect(labels).toEqual(LANGUAGE_REGISTRY.map(({ label }) => label));
+  });
+
   test('keeps every folded settings concern visible on one page', () => {
     (axios.get as jest.Mock).mockResolvedValue({ data: { hasToken: false } });
     renderSettings();

frontend/src/v2/__tests__/languageSurfaces.test.tsx

@@ -0,0 +1,122 @@
+import React from 'react';
+import { act, fireEvent, render, screen, within } from '@testing-library/react';
+import { MemoryRouter } from 'react-router-dom';
+import axios from 'axios';
+import i18n, { i18nReady, LANGUAGE_REGISTRY, LANGUAGE_STORAGE_KEY } from '../../i18n';
+import { useAuth } from '../../context/AuthContext';
+import V2LangSwitch from '../components/V2LangSwitch';
+import V2NavRail from '../components/V2NavRail';
+import V2LandingPage from '../landing/V2LandingPage';
+
+jest.mock('axios');
+jest.mock('../../context/AuthContext', () => ({
+  useAuth: jest.fn(),
+}));
+jest.mock('../hooks/useV2PodAttention', () => ({
+  useV2PodAttention: () => ({ attentionByPod: {}, totalNeedsYou: 0 }),
+}));
+jest.mock('../components/V2AccountMenu', () => {
+  const MockV2AccountMenu = () => <div>account menu</div>;
+  MockV2AccountMenu.displayName = 'MockV2AccountMenu';
+  return { __esModule: true, default: MockV2AccountMenu };
+});
+jest.mock('../components/V2FeedbackMenu', () => {
+  const MockV2FeedbackMenu = () => <div>feedback menu</div>;
+  MockV2FeedbackMenu.displayName = 'MockV2FeedbackMenu';
+  return { __esModule: true, default: MockV2FeedbackMenu };
+});
+
+// Written out on purpose, NOT derived from LANGUAGE_REGISTRY. TASK-318 left one
+// registry feeding the switcher, Settings and <html lang>, and its own check
+// compared SUPPORTED_LANGUAGES to the same map that defines it — an assertion
+// that moves with whatever it is meant to pin (Sprint Review 5611893458).
+// Changing the shipped languages has to fail here first, so that whoever adds
+// one is sent through every surface below.
+const EXPECTED_LANGUAGES = [
+  { code: 'en', label: 'English', short: 'EN' },
+  { code: 'zh-CN', label: '中文', short: '中文' },
+];
+
+const mockUseAuth = useAuth as jest.Mock;
+const mockAxiosGet = axios.get as jest.Mock;
+
+// The trigger and menu are labelled through i18n, so their accessible names
+// change with the active language. Role alone is the stable handle here.
+const openSwitcherMenu = (trigger: HTMLElement) => {
+  fireEvent.click(trigger);
+  return screen.getByRole('listbox');
+};
+
+describe('language surfaces derive from the registry', () => {
+  beforeAll(async () => {
+    await i18nReady;
+  });
+
+  beforeEach(async () => {
+    jest.clearAllMocks();
+    mockUseAuth.mockReturnValue({ isAuthenticated: false, user: null });
+    mockAxiosGet.mockResolvedValue({ data: {} });
+    localStorage.removeItem(LANGUAGE_STORAGE_KEY);
+    await act(async () => {
+      await i18n.changeLanguage('en');
+    });
+  });
+
+  afterAll(async () => {
+    localStorage.removeItem(LANGUAGE_STORAGE_KEY);
+    await i18n.changeLanguage('en');
+  });
+
+  it('ships exactly the pinned languages', () => {
+    expect(LANGUAGE_REGISTRY.map(({ code, label, short }) => ({ code, label, short })))
+      .toEqual(EXPECTED_LANGUAGES);
+  });
+
+  it('offers every registered language in the switcher menu, and nothing else', () => {
+    render(<V2LangSwitch />);
+
+    const options = within(openSwitcherMenu(screen.getByRole('button', { name: 'Language' })))
+      .getAllByRole('option');
+    expect(options.map((option) => option.textContent?.replace('✓', '').trim()))
+      .toEqual(EXPECTED_LANGUAGES.map(({ label }) => label));
+  });
+
+  it('switches to each registered language from the switcher menu', async () => {
+    render(<V2LangSwitch />);
+
+    const trigger = screen.getByRole('button', { name: 'Language' });
+
+    for (const { code, label, short } of EXPECTED_LANGUAGES) {
+      // The trigger click must settle before the option is queried, so opening
+      // the menu stays outside act() — fireEvent already flushes it.
+      const option = within(openSwitcherMenu(trigger)).getByRole('button', { name: label });
+      // eslint-disable-next-line no-await-in-loop
+      await act(async () => {
+        fireEvent.click(option);
+      });
+      expect(i18n.resolvedLanguage).toBe(code);
+      expect(document.documentElement.lang).toBe(code);
+      expect(trigger).toHaveTextContent(short);
+    }
+  });
+
+  it('keeps the language menu on the rail', () => {
+    render(
+      <MemoryRouter>
+        <V2NavRail needsYouCount={0} />
+      </MemoryRouter>,
+    );
+
+    expect(screen.getByRole('button', { name: 'Language' })).toBeInTheDocument();
+  });
+
+  it('keeps the language menu on the landing nav', () => {
+    render(
+      <MemoryRouter>
+        <V2LandingPage />
+      </MemoryRouter>,
+    );
+
+    expect(screen.getAllByRole('button', { name: 'Language' }).length).toBeGreaterThan(0);
+  });
+});