diff --git a/apps/desktop/DESIGN.md b/apps/desktop/DESIGN.md
index 26fabb5b57..2d37178fb1 100644
--- a/apps/desktop/DESIGN.md
+++ b/apps/desktop/DESIGN.md
@@ -487,9 +487,12 @@ long transcript or a busy terminal.
 
 - Every user-facing string goes through `useI18n()` (`src/i18n/context.tsx`).
   No literals in JSX.
-- **Update all locales together** — `en`, `ja`, `zh`, `zh-hant`. A string change
-  in `en.ts` that skips the others is a regression (drifted punctuation,
-  stale labels). Keep trailing-punctuation and tone consistent across all four.
+- **Update all locales together** — every catalog registered in
+  `src/i18n/catalog.ts`. A string change in `en.ts` that skips the others is a
+  regression (drifted punctuation, stale labels). Keep trailing-punctuation and
+  tone consistent across all of them. `fr`, `de`, and `es` are complete
+  `Translations` objects, so a key missing there fails the type check; the
+  `defineLocale()` overlays fall back to English instead.
 
 ## State (TypeScript)
 
@@ -535,7 +538,7 @@ The detailed state contract lives in the scoped
 - [ ] Hot interactions avoid broad subscriptions, layout thrash, and
       `transition-all`?
 - [ ] Keyboard ownership and single-action `Esc` behavior are correct?
-- [ ] All four locales updated for any new/changed string?
+- [ ] All registered locales updated for any new/changed string?
 - [ ] `cursor-pointer`, focus ring, and `Esc`-to-close behave?
 - [ ] Touched a primitive, token, or variant? Its named-contract entry in this
       file is updated in the same change.
diff --git a/apps/desktop/src/app/chat/composer/hooks/slash-localization.test.tsx b/apps/desktop/src/app/chat/composer/hooks/slash-localization.test.tsx
index 19529f7d66..68c982754a 100644
--- a/apps/desktop/src/app/chat/composer/hooks/slash-localization.test.tsx
+++ b/apps/desktop/src/app/chat/composer/hooks/slash-localization.test.tsx
@@ -114,7 +114,7 @@ describe('desktop slash description localization', () => {
     )
 
     for (const query of ['', 'ne']) {
-      for (const locale of ['en', 'zh', 'ja', 'zh-hant', 'ar', 'ru', 'en'] as const) {
+      for (const locale of ['en', 'zh', 'ja', 'zh-hant', 'ar', 'ru', 'fr', 'de', 'es', 'en'] as const) {
         await act(async () => {
           await api.setLocale!(locale)
         })
diff --git a/apps/desktop/src/components/chat/intro.test.tsx b/apps/desktop/src/components/chat/intro.test.tsx
index a98090987b..9945146795 100644
--- a/apps/desktop/src/components/chat/intro.test.tsx
+++ b/apps/desktop/src/components/chat/intro.test.tsx
@@ -7,6 +7,8 @@ import type { I18nContextValue } from '@/i18n'
 import { Intro } from './intro'
 import stock from './intro-copy.jsonl?raw'
 
+const CJK_LOCALES = new Set(['zh', 'zh-hant', 'ja'])
+
 let i18n: I18nContextValue
 
 function Controls() {
@@ -38,7 +40,7 @@ it('translates every shipped stock body at the same personality and rotation pos
 
   const { container, rerender } = render(<Fixture />)
 
-  for (const locale of ['zh', 'zh-hant', 'ja'] as const) {
+  for (const locale of ['zh', 'zh-hant', 'ja', 'fr', 'de', 'es'] as const) {
     await act(() => i18n.setLocale(locale))
     const indices = new Map<string, number>()
 
@@ -55,7 +57,10 @@ it('translates every shipped stock body at the same personality and rotation pos
       const body = container.querySelector('[data-slot="aui_intro"] > div > p:last-child')!.textContent
       expect(body).toBeTruthy()
       expect(body).not.toBe(entry.body)
-      expect(body).toMatch(/[\u3040-\u30ff\u3400-\u9fff]/)
+
+      if (CJK_LOCALES.has(locale)) {
+        expect(body).toMatch(/[\u3040-\u30ff\u3400-\u9fff]/)
+      }
     }
   }
 
@@ -76,4 +81,11 @@ it('localizes the custom-personality fallback without translating its user-suppl
   expect(container.querySelector('[data-slot="aui_intro"] > div > p:last-child')!.textContent).toContain(
     'My Custom Voice'
   )
+
+  for (const locale of ['fr', 'de', 'es'] as const) {
+    await act(() => i18n.setLocale(locale))
+    const body = container.querySelector('[data-slot="aui_intro"] > div > p:last-child')!.textContent
+    expect(body).not.toBe(english)
+    expect(body).toContain('My Custom Voice')
+  }
 })
diff --git a/apps/desktop/src/i18n/catalog-completeness.test.ts b/apps/desktop/src/i18n/catalog-completeness.test.ts
new file mode 100644
index 0000000000..ad8479b06c
--- /dev/null
+++ b/apps/desktop/src/i18n/catalog-completeness.test.ts
@@ -0,0 +1,84 @@
+import { describe, expect, it } from 'vitest'
+
+import { TRANSLATIONS } from './catalog'
+import type { Locale } from './types'
+
+// Locales shipped as full catalogs (not `defineLocale` overlays), so every
+// English key must be present in the catalog object itself.
+const COMPLETE_LOCALES = ['fr', 'de', 'es'] as const satisfies readonly Locale[]
+
+type Leaf = { path: string; value: unknown }
+
+function leaves(value: unknown, path = ''): Leaf[] {
+  if (value && typeof value === 'object' && !Array.isArray(value)) {
+    return Object.entries(value).flatMap(([key, child]) => leaves(child, path ? `${path}.${key}` : key))
+  }
+
+  return [{ path, value }]
+}
+
+// Arguments that are identifiers rather than display text; translations
+// branch on them (`capability === 'search' ? … : …`) instead of echoing them.
+const IDENTIFIER_ARGS: Record<string, number[]> = {
+  'settings.toolsets.webCapabilitySelectedMessage': [1]
+}
+
+const kindOf = (value: unknown) => (Array.isArray(value) ? 'array' : typeof value)
+
+// `intro` is display-only: English lives in intro-copy.jsonl, so its catalog
+// entry is an empty shell. intro.test.tsx covers the translated rotation.
+const catalogLeaves = (locale: Locale) =>
+  new Map(
+    leaves(TRANSLATIONS[locale])
+      .filter(leaf => !leaf.path.startsWith('intro.'))
+      .map(leaf => [leaf.path, leaf.value])
+  )
+
+const english = catalogLeaves('en')
+
+describe.each(COMPLETE_LOCALES)('%s desktop catalog', locale => {
+  const catalog = catalogLeaves(locale)
+
+  it('covers exactly the English key set with matching value kinds', () => {
+    expect([...catalog.keys()].sort()).toEqual([...english.keys()].sort())
+
+    for (const [path, value] of english) {
+      expect({ path, kind: kindOf(catalog.get(path)) }).toEqual({ path, kind: kindOf(value) })
+    }
+  })
+
+  it('keeps every interpolated argument that English renders', () => {
+    for (const [path, value] of english) {
+      if (typeof value !== 'function') {
+        continue
+      }
+
+      const translated = catalog.get(path) as (...args: unknown[]) => unknown
+      const probes = Array.from({ length: value.length }, (_, index) => `⟦${index}⟧`)
+      let englishOut: string
+
+      try {
+        englishOut = JSON.stringify(value(...probes))
+      } catch {
+        continue // needs structured arguments; the type checker covers the signature
+      }
+
+      expect(translated.length, path).toBe(value.length)
+      const translatedOut = JSON.stringify(translated(...probes))
+
+      const identifiers = new Set(IDENTIFIER_ARGS[path]?.map(index => probes[index]))
+
+      for (const probe of probes.filter(probe => englishOut.includes(probe) && !identifiers.has(probe))) {
+        expect(translatedOut, `${path} drops ${probe}`).toContain(probe)
+      }
+    }
+  })
+
+  it('keeps list-shaped copy the same length as English', () => {
+    for (const [path, value] of english) {
+      if (Array.isArray(value)) {
+        expect((catalog.get(path) as unknown[]).length, path).toBe(value.length)
+      }
+    }
+  })
+})
diff --git a/apps/desktop/src/i18n/catalog.ts b/apps/desktop/src/i18n/catalog.ts
index 220a88b7f2..02c45d0a1c 100644
--- a/apps/desktop/src/i18n/catalog.ts
+++ b/apps/desktop/src/i18n/catalog.ts
@@ -1,5 +1,8 @@
 import { ar } from './ar'
+import { de } from './de'
 import { en } from './en'
+import { es } from './es'
+import { fr } from './fr'
 import { ja } from './ja'
 import { ru } from './ru'
 import type { Locale, Translations } from './types'
@@ -12,5 +15,8 @@ export const TRANSLATIONS: Record<Locale, Translations> = {
   'zh-hant': zhHant,
   ja,
   ar,
-  ru
+  ru,
+  fr,
+  de,
+  es
 }
diff --git a/apps/desktop/src/i18n/context.test.tsx b/apps/desktop/src/i18n/context.test.tsx
index 2b24ca31e5..4790107cfe 100644
--- a/apps/desktop/src/i18n/context.test.tsx
+++ b/apps/desktop/src/i18n/context.test.tsx
@@ -3,6 +3,7 @@ import { afterEach, describe, expect, it, vi } from 'vitest'
 
 import type { HermesConfigRecord } from '@/hermes'
 
+import { TRANSLATIONS } from './catalog'
 import { type I18nConfigClient, I18nProvider, useI18n } from './context'
 import type { Locale } from './types'
 
@@ -76,6 +77,31 @@ describe('I18nProvider', () => {
     expect(configClient.saveConfig).not.toHaveBeenCalled()
   })
 
+  it.each([
+    ['fr', 'fr'],
+    ['de-DE', 'de'],
+    ['es', 'es']
+  ] as const)('loads display.language=%s and renders the %s catalog', async (configured, locale) => {
+    const configClient: I18nConfigClient = {
+      getConfig: vi.fn().mockResolvedValue({ display: { language: configured } }),
+      saveConfig: vi.fn()
+    }
+
+    render(
+      <I18nProvider configClient={configClient}>
+        <LanguageProbe />
+      </I18nProvider>
+    )
+
+    await waitFor(() => expect(screen.getByTestId('loading').textContent).toBe('false'))
+
+    expect(screen.getByTestId('locale').textContent).toBe(locale)
+    expect(screen.getByTestId('label').textContent).toBe(TRANSLATIONS[locale].language.label)
+    expect(screen.getByTestId('label').textContent).not.toBe(TRANSLATIONS.en.language.label)
+    expect(screen.getByTestId('save').textContent).not.toBe(TRANSLATIONS.en.common.save)
+    expect(configClient.saveConfig).not.toHaveBeenCalled()
+  })
+
   it('keeps English usable when config loading fails', async () => {
     const configClient: I18nConfigClient = {
       getConfig: vi.fn().mockRejectedValue(new Error('config unavailable')),
@@ -97,7 +123,7 @@ describe('I18nProvider', () => {
 
   it('does not overwrite unsupported configured languages', async () => {
     const configClient: I18nConfigClient = {
-      getConfig: vi.fn().mockResolvedValue({ display: { language: 'de' } }),
+      getConfig: vi.fn().mockResolvedValue({ display: { language: 'it' } }),
       saveConfig: vi.fn()
     }
 
diff --git a/apps/desktop/src/i18n/languages.test.ts b/apps/desktop/src/i18n/languages.test.ts
index 7648a1b427..f2692d30e8 100644
--- a/apps/desktop/src/i18n/languages.test.ts
+++ b/apps/desktop/src/i18n/languages.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
 
-import { DEFAULT_LOCALE, isLocale, isSupportedLocaleValue, normalizeLocale } from './languages'
+import { TRANSLATIONS } from './catalog'
+import { DEFAULT_LOCALE, isLocale, isSupportedLocaleValue, LOCALE_OPTIONS, normalizeLocale } from './languages'
 
 describe('desktop i18n languages', () => {
   it('normalizes supported locale aliases', () => {
@@ -22,12 +23,24 @@ describe('desktop i18n languages', () => {
     expect(normalizeLocale('RU-RU')).toBe('ru')
     expect(normalizeLocale(' ru_ru ')).toBe('ru')
     expect(normalizeLocale('Русский')).toBe('ru')
+    expect(normalizeLocale('fr')).toBe('fr')
+    expect(normalizeLocale('FR-CA')).toBe('fr')
+    expect(normalizeLocale(' fr_fr ')).toBe('fr')
+    expect(normalizeLocale('Français')).toBe('fr')
+    expect(normalizeLocale('de')).toBe('de')
+    expect(normalizeLocale('DE-AT')).toBe('de')
+    expect(normalizeLocale(' de_ch ')).toBe('de')
+    expect(normalizeLocale('Deutsch')).toBe('de')
+    expect(normalizeLocale('es')).toBe('es')
+    expect(normalizeLocale('ES-419')).toBe('es')
+    expect(normalizeLocale(' es_mx ')).toBe('es')
+    expect(normalizeLocale('Español')).toBe('es')
   })
 
   it('falls back to English for empty or unsupported values', () => {
     expect(normalizeLocale(null)).toBe(DEFAULT_LOCALE)
     expect(normalizeLocale('')).toBe(DEFAULT_LOCALE)
-    expect(normalizeLocale('de')).toBe(DEFAULT_LOCALE)
+    expect(normalizeLocale('it')).toBe(DEFAULT_LOCALE)
   })
 
   it('distinguishes exact locale ids from supported config aliases', () => {
@@ -35,12 +48,25 @@ describe('desktop i18n languages', () => {
     expect(isSupportedLocaleValue('zh-TW')).toBe(true)
     expect(isSupportedLocaleValue('ja-JP')).toBe(true)
     expect(isSupportedLocaleValue('ru-RU')).toBe(true)
-    expect(isSupportedLocaleValue('de')).toBe(false)
+    expect(isSupportedLocaleValue('de-DE')).toBe(true)
+    expect(isSupportedLocaleValue('it')).toBe(false)
     expect(isLocale('zh-CN')).toBe(false)
     expect(isLocale('zh')).toBe(true)
     expect(isLocale('zh-hant')).toBe(true)
     expect(isLocale('ja')).toBe(true)
     expect(isLocale('ar')).toBe(true)
     expect(isLocale('ru')).toBe(true)
+    expect(isLocale('fr')).toBe(true)
+    expect(isLocale('de')).toBe(true)
+    expect(isLocale('es')).toBe(true)
+  })
+
+  it('round-trips every picker option through its display.language value to a registered catalog', () => {
+    for (const option of LOCALE_OPTIONS) {
+      expect(normalizeLocale(option.configValue)).toBe(option.id)
+      expect(TRANSLATIONS[option.id]).toBeDefined()
+    }
+
+    expect(Object.keys(TRANSLATIONS).sort()).toEqual(LOCALE_OPTIONS.map(option => option.id).sort())
   })
 })
diff --git a/apps/desktop/src/i18n/languages.ts b/apps/desktop/src/i18n/languages.ts
index 778a664869..83cac13045 100644
--- a/apps/desktop/src/i18n/languages.ts
+++ b/apps/desktop/src/i18n/languages.ts
@@ -42,6 +42,24 @@ export const LOCALE_OPTIONS = [
     name: LOCALE_ENDONYMS.ru,
     englishName: 'Russian',
     configValue: 'ru'
+  },
+  {
+    id: 'fr',
+    name: LOCALE_ENDONYMS.fr,
+    englishName: 'French',
+    configValue: 'fr'
+  },
+  {
+    id: 'de',
+    name: LOCALE_ENDONYMS.de,
+    englishName: 'German',
+    configValue: 'de'
+  },
+  {
+    id: 'es',
+    name: LOCALE_ENDONYMS.es,
+    englishName: 'Spanish',
+    configValue: 'es'
   }
 ] as const satisfies readonly { configValue: string; englishName: string; id: Locale; name: string }[]
 
@@ -96,7 +114,40 @@ const LOCALE_ALIASES: Record<string, Locale> = {
   russian: 'ru',
   'russian-russian': 'ru',
   русский: 'ru',
-  руский: 'ru'
+  руский: 'ru',
+  fr: 'fr',
+  'fr-fr': 'fr',
+  fr_fr: 'fr',
+  'fr-be': 'fr',
+  fr_be: 'fr',
+  'fr-ca': 'fr',
+  fr_ca: 'fr',
+  'fr-ch': 'fr',
+  fr_ch: 'fr',
+  french: 'fr',
+  français: 'fr',
+  francais: 'fr',
+  de: 'de',
+  'de-de': 'de',
+  de_de: 'de',
+  'de-at': 'de',
+  de_at: 'de',
+  'de-ch': 'de',
+  de_ch: 'de',
+  german: 'de',
+  deutsch: 'de',
+  es: 'es',
+  'es-es': 'es',
+  es_es: 'es',
+  'es-mx': 'es',
+  es_mx: 'es',
+  'es-ar': 'es',
+  es_ar: 'es',
+  'es-419': 'es',
+  es_419: 'es',
+  spanish: 'es',
+  español: 'es',
+  espanol: 'es'
 }
 
 export function isLocale(value: unknown): value is Locale {
diff --git a/apps/desktop/src/i18n/types.ts b/apps/desktop/src/i18n/types.ts
index 04d2b55077..3ba6192076 100644
--- a/apps/desktop/src/i18n/types.ts
+++ b/apps/desktop/src/i18n/types.ts
@@ -8,7 +8,7 @@
 import type { ErrorCodeKey } from '@/lib/error-surface'
 import type { TipId } from '@/lib/tips/catalog'
 
-export type Locale = 'en' | 'zh' | 'zh-hant' | 'ja' | 'ar' | 'ru'
+export type Locale = 'en' | 'zh' | 'zh-hant' | 'ja' | 'ar' | 'ru' | 'fr' | 'de' | 'es'
 
 /** One error-card entry: a short title and one plain sentence. Either may
  *  take the failing provider's display name (falls back to "the AI service"). */