diff --git a/frontend/src/lib/__tests__/i18n.test.ts b/frontend/src/lib/__tests__/i18n.test.ts
index 8f0776b8..bb6c38e7 100644
--- a/frontend/src/lib/__tests__/i18n.test.ts
+++ b/frontend/src/lib/__tests__/i18n.test.ts
@@ -22,6 +22,23 @@ describe('i18n', () => {
       i18n.setLocale('invalid' as any);
       expect(i18n.getLocale()).toBe('en');
     });
+
+    it('should return true when setting an implemented locale', () => {
+      const result = i18n.setLocale('en');
+      expect(result).toBe(true);
+    });
+
+    it('should return false for unimplemented locales (es, fr, de)', () => {
+      expect(i18n.setLocale('es')).toBe(false);
+      expect(i18n.setLocale('fr')).toBe(false);
+      expect(i18n.setLocale('de')).toBe(false);
+    });
+
+    it('should not change internal locale when an unimplemented locale is requested', () => {
+      i18n.setLocale('en');
+      i18n.setLocale('es');
+      expect(i18n.getLocale()).toBe('en');
+    });
   });
 
   describe('t (translation)', () => {
diff --git a/frontend/src/lib/hooks/__tests__/useI18n.test.ts b/frontend/src/lib/hooks/__tests__/useI18n.test.ts
new file mode 100644
index 00000000..c2d4ca5b
--- /dev/null
+++ b/frontend/src/lib/hooks/__tests__/useI18n.test.ts
@@ -0,0 +1,128 @@
+import { renderHook, act, waitFor } from '@testing-library/react';
+import { useI18n } from '../useI18n';
+import { i18n } from '../../i18n';
+
+describe('useI18n', () => {
+  beforeEach(() => {
+    // Reset singleton and storage before each test
+    i18n.setLocale('en');
+    localStorage.clear();
+  });
+
+  it('initialises with en locale', async () => {
+    const { result } = renderHook(() => useI18n());
+
+    await waitFor(() => expect(result.current.locale).toBe('en'));
+    expect(i18n.getLocale()).toBe('en');
+  });
+
+  describe('setLocale with implemented locale (en)', () => {
+    it('updates React state when locale is implemented', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('en');
+      });
+
+      expect(result.current.locale).toBe('en');
+      expect(i18n.getLocale()).toBe('en');
+    });
+
+    it('keeps React state and singleton in sync for en', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('en');
+      });
+
+      // Both sides agree
+      expect(result.current.locale).toBe(i18n.getLocale());
+    });
+  });
+
+  describe('setLocale with unimplemented locales (es, fr, de)', () => {
+    it('does NOT update React state when locale is unimplemented (es)', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('es');
+      });
+
+      // React state must stay at 'en' — no desync
+      expect(result.current.locale).toBe('en');
+    });
+
+    it('does NOT update React state when locale is unimplemented (fr)', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('fr');
+      });
+
+      expect(result.current.locale).toBe('en');
+    });
+
+    it('does NOT update React state when locale is unimplemented (de)', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('de');
+      });
+
+      expect(result.current.locale).toBe('en');
+    });
+
+    it('keeps React state and i18n singleton in sync when unimplemented locale is requested', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('es');
+      });
+
+      // The core invariant: hook-visible locale === what the singleton actually uses
+      expect(result.current.locale).toBe(i18n.getLocale());
+    });
+
+    it('singleton internal locale stays en after unimplemented locale request', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      act(() => {
+        result.current.setLocale('fr');
+      });
+
+      expect(i18n.getLocale()).toBe('en');
+    });
+  });
+
+  describe('t() translations', () => {
+    it('returns translation for a known key', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      expect(result.current.t('nav.features')).toBe('Features');
+    });
+
+    it('returns fallback for unknown key', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      expect(result.current.t('nonexistent.key', 'fallback')).toBe('fallback');
+    });
+  });
+
+  describe('availableLocales', () => {
+    it('returns at least en', async () => {
+      const { result } = renderHook(() => useI18n());
+      await waitFor(() => expect(result.current.locale).toBe('en'));
+
+      expect(result.current.availableLocales).toContain('en');
+    });
+  });
+});
diff --git a/frontend/src/lib/hooks/useI18n.ts b/frontend/src/lib/hooks/useI18n.ts
index 300d2063..b3886a8b 100644
--- a/frontend/src/lib/hooks/useI18n.ts
+++ b/frontend/src/lib/hooks/useI18n.ts
@@ -13,8 +13,10 @@ export function useI18n() {
   }, []);
 
   const setLocale = (newLocale: Locale) => {
-    i18n.setLocale(newLocale);
-    setLocaleState(newLocale);
+    const applied = i18n.setLocale(newLocale);
+    if (applied) {
+      setLocaleState(newLocale);
+    }
   };
 
   const t = (key: string, defaultValue?: string) => {
diff --git a/frontend/src/lib/i18n.ts b/frontend/src/lib/i18n.ts
index 92eefea6..a5706f79 100644
--- a/frontend/src/lib/i18n.ts
+++ b/frontend/src/lib/i18n.ts
@@ -90,13 +90,15 @@ const translations: LocaleData = {
 class I18n {
   private currentLocale: Locale = 'en';
 
-  setLocale(locale: Locale): void {
+  setLocale(locale: Locale): boolean {
     if (locale in translations) {
       this.currentLocale = locale;
       if (typeof window !== 'undefined') {
         localStorage.setItem('locale', locale);
       }
+      return true;
     }
+    return false;
   }
 
   getLocale(): Locale {