diff --git a/apps/desktop/src/i18n/ar.ts b/apps/desktop/src/i18n/ar.ts
index a0c2fe06c05d..5bf6480fb4c3 100644
--- a/apps/desktop/src/i18n/ar.ts
+++ b/apps/desktop/src/i18n/ar.ts
@@ -232,7 +232,6 @@ export const ar = defineLocale({
       'view.terminalSelection': 'إرسال تحديد الطرفية إلى المحرّر',
       'view.terminalCopy': 'نسخ تحديد الطرفية',
       'view.terminalPaste': 'لصق في الطرفية',
-      'view.closePreviewTab': 'إغلاق علامة تبويب المعاينة',
       'view.flipPanes': 'تبديل جانبي الشريط الجانبي',
       'appearance.toggleMode': 'تبديل الفاتح / الداكن',
       'profile.default': 'الانتقال إلى الملف الشخصي الافتراضي',
@@ -1957,7 +1956,6 @@ export const ar = defineLocale({
     flowSubtitles: {
       pkce: 'يفتح المتصفح لتسجيل الدخول ثم يتابع هنا',
       device_code: 'يفتح صفحة تحقق في المتصفح — يتصل Hermes تلقائياً',
-      loopback: 'يفتح المتصفح لتسجيل الدخول — يتصل Hermes تلقائياً',
       external: 'سجل الدخول مرة واحدة في الطرفية ثم عد إلى المحادثة'
     },
     startingSignIn: provider => `جار بدء تسجيل الدخول لـ ${provider}...`,
diff --git a/apps/desktop/src/i18n/locale-parity.test.ts b/apps/desktop/src/i18n/locale-parity.test.ts
new file mode 100644
index 000000000000..373699f48ea4
--- /dev/null
+++ b/apps/desktop/src/i18n/locale-parity.test.ts
@@ -0,0 +1,133 @@
+import { describe, expect, it } from 'vitest'
+
+import { TRANSLATIONS } from './catalog'
+import { en } from './en'
+
+/**
+ * Locale parity guard.
+ *
+ * `defineLocale()` deep-merges a locale's overrides onto `en`, and the
+ * override type makes every key optional. That is deliberate - a partially
+ * translated locale should fall back to English rather than break.
+ *
+ * The failure mode it hides is a locale key that English NO LONGER HAS.
+ * TypeScript cannot catch it: blocks like `composer.commandDescs` are typed
+ * `Record<string, string>`, so any key satisfies the type. Verified against
+ * this repo - re-adding a removed key to a locale leaves `tsc --noEmit`
+ * exiting 0.
+ *
+ * That is not cosmetic. #66646 removed terminal-only commands (`/clear`,
+ * `/details`, `/copy`, `/quit`) from the desktop quick-help drawer because
+ * the desktop refuses them, but a locale that still defined those keys kept
+ * advertising commands that do not work. The stale entries survive the merge
+ * and win over English, so the very bug the change fixed stayed live in that
+ * language while every other locale was corrected.
+ *
+ * This guard fails on exactly that: a key defined by a locale that does not
+ * exist in English. Missing keys are NOT flagged - falling back is the
+ * design.
+ */
+
+/**
+ * Blocks whose keys are supplied at runtime rather than fixed by English.
+ *
+ * `sidebar.nav` is a contribution registry (`SIDEBAR_NAV_AREA` in
+ * `app/routes.ts`) - plugins register nav items, so a locale carrying a label
+ * for a contributed item is legitimate rather than stale. Keep this list
+ * short: every entry is a place the guard cannot protect.
+ */
+const REGISTRY_PATHS = new Set(['sidebar.nav'])
+
+type Node = Record<string, unknown>
+
+const isRecord = (value: unknown): value is Node =>
+  typeof value === 'object' && value !== null && !Array.isArray(value)
+
+/** Key paths present in `node` but absent from `reference`, depth-first. */
+function extraKeyPaths(node: unknown, reference: unknown, trail: string[] = []): string[] {
+  if (!isRecord(node) || !isRecord(reference)) {
+    return []
+  }
+
+  const found: string[] = []
+
+  // A block English leaves deliberately empty (e.g. `platformIntro: {}`) is an
+  // extension point locales are meant to populate, not a set they must match.
+  if (Object.keys(reference).length === 0) {
+    return []
+  }
+
+  if (REGISTRY_PATHS.has(trail.join('.'))) {
+    return []
+  }
+
+  for (const [key, value] of Object.entries(node)) {
+    const path = [...trail, key]
+
+    if (!(key in reference)) {
+      found.push(path.join('.'))
+      continue
+    }
+
+    found.push(...extraKeyPaths(value, reference[key], path))
+  }
+
+  return found
+}
+
+describe('locale parity', () => {
+  const locales = Object.entries(TRANSLATIONS).filter(([id]) => id !== 'en')
+
+  it('covers every registered locale', () => {
+    // A locale added to the catalog without being added here would silently
+    // escape the guard, which is the same class of miss this test exists for.
+    expect(locales.length).toBeGreaterThan(0)
+    expect(Object.keys(TRANSLATIONS)).toContain('en')
+  })
+
+  it.each(locales)('%s defines no key English has dropped', (id, translations) => {
+    const stale = extraKeyPaths(translations, en)
+
+    expect(
+      stale,
+      stale.length
+        ? `${id} defines ${stale.length} key(s) English no longer has, so they win over the ` +
+            `English value and can surface removed or renamed UI:\n  ${stale.join('\n  ')}`
+        : ''
+    ).toEqual([])
+  })
+})
+
+describe('extraKeyPaths', () => {
+  it('reports a key the reference does not have', () => {
+    expect(extraKeyPaths({ a: 1, gone: 2 }, { a: 1 })).toEqual(['gone'])
+  })
+
+  it('reports nested keys with a dotted trail', () => {
+    expect(extraKeyPaths({ outer: { kept: 1, stale: 2 } }, { outer: { kept: 1 } })).toEqual([
+      'outer.stale'
+    ])
+  })
+
+  it('does not report missing keys - falling back to English is by design', () => {
+    expect(extraKeyPaths({ a: 1 }, { a: 1, translatedLater: 2 })).toEqual([])
+  })
+
+  it('ignores a block English leaves deliberately empty', () => {
+    // `en.messaging.platformIntro` is `{}` on purpose - locales fill it.
+    expect(extraKeyPaths({ platformIntro: { slack: 'x' } }, { platformIntro: {} })).toEqual([])
+  })
+
+  it('ignores a documented runtime registry block', () => {
+    // Plugins contribute sidebar nav items, so extra locale labels there are
+    // not evidence of a removed key.
+    expect(
+      extraKeyPaths({ nav: { plugin: 'x' } }, { nav: { core: 'y' } }, ['sidebar'])
+    ).toEqual([])
+  })
+
+  it('ignores functions and arrays rather than walking into them', () => {
+    const fn = (n: number) => `${n}`
+    expect(extraKeyPaths({ fn, list: [1, 2] }, { fn, list: [1] })).toEqual([])
+  })
+})