diff --git a/src/workspaces/i18n/src/I18nProvider.tsx b/src/workspaces/i18n/src/I18nProvider.tsx
index 41c66a74..b5f15755 100644
--- a/src/workspaces/i18n/src/I18nProvider.tsx
+++ b/src/workspaces/i18n/src/I18nProvider.tsx
@@ -11,15 +11,19 @@ export const I18nContext = createContext<I18nContextValue>({
 type I18nProviderProps = {
   translations: TranslationMap;
   locale: string;
+  /** The catalog `resolveKey` falls back to on a miss — a bug path, per D8, not a regional chain. */
+  fallbackLocale?: string;
   children: ReactNode;
 };
 
-export const I18nProvider = ({ translations, locale, children }: I18nProviderProps) => {
+export const I18nProvider = ({ translations, locale, fallbackLocale = 'en', children }: I18nProviderProps) => {
   const value = useMemo<I18nContextValue>(() => {
-    const t = (key: string, params?: InterpolationParams) =>
-      interpolate(resolveKey(translations, locale, key), params);
+    const t = (key: string, params?: InterpolationParams) => {
+      const count = typeof params?.count === 'number' ? params.count : undefined;
+      return interpolate(resolveKey(translations, locale, key, fallbackLocale, count), params);
+    };
     return { t, locale };
-  }, [translations, locale]);
+  }, [translations, locale, fallbackLocale]);
 
   return <I18nContext.Provider value={value}>{children}</I18nContext.Provider>;
 };
diff --git a/src/workspaces/i18n/src/index.ts b/src/workspaces/i18n/src/index.ts
index ff005cfc..14777e5c 100644
--- a/src/workspaces/i18n/src/index.ts
+++ b/src/workspaces/i18n/src/index.ts
@@ -1,4 +1,7 @@
 export { I18nProvider, I18nContext } from './I18nProvider';
 export { useTranslation } from './useTranslation';
-export { resolveKey, interpolate } from './resolve';
-export type { TranslationMap, InterpolationParams, I18nContextValue, TranslationKeys } from './types';
+export { useFormat } from './useFormat';
+export { resolveKey, interpolate, isPluralForms, localeDirection } from './resolve';
+export { findMissingKeys } from './parity';
+export type { MissingKey } from './parity';
+export type { TranslationMap, InterpolationParams, I18nContextValue, TranslationKeys, PluralForms } from './types';
diff --git a/src/workspaces/i18n/src/parity.ts b/src/workspaces/i18n/src/parity.ts
new file mode 100644
index 00000000..286cb408
--- /dev/null
+++ b/src/workspaces/i18n/src/parity.ts
@@ -0,0 +1,27 @@
+import { isPluralForms } from './resolve';
+
+type Catalog = Record<string, unknown>;
+
+const flattenKeys = (obj: Catalog, prefix = ''): string[] =>
+  Object.entries(obj).flatMap(([k, v]) => {
+    const path = prefix ? `${prefix}.${k}` : k;
+    if (v !== null && typeof v === 'object' && !Array.isArray(v) && !isPluralForms(v)) {
+      return flattenKeys(v as Catalog, path);
+    }
+    return [path];
+  });
+
+export type MissingKey = { locale: string; key: string };
+
+/**
+ * Per D2, every shipped locale is a COMPLETE catalog — no regional inheritance. So a key `en` has
+ * and a locale's own file lacks is not a fallback case, it's a red build (docs/localization.md,
+ * "Tooling and guardrails").
+ */
+export const findMissingKeys = (en: Catalog, locales: Record<string, Catalog>): MissingKey[] => {
+  const enKeys = flattenKeys(en);
+  return Object.entries(locales).flatMap(([locale, catalog]) => {
+    const ownKeys = new Set(flattenKeys(catalog));
+    return enKeys.filter((key) => !ownKeys.has(key)).map((key) => ({ locale, key }));
+  });
+};
diff --git a/src/workspaces/i18n/src/resolve.test.ts b/src/workspaces/i18n/src/resolve.test.ts
new file mode 100644
index 00000000..4338050c
--- /dev/null
+++ b/src/workspaces/i18n/src/resolve.test.ts
@@ -0,0 +1,102 @@
+import { describe, expect, test } from 'bun:test';
+import { resolveKey, interpolate, isPluralForms, localeDirection } from './resolve';
+import { findMissingKeys } from './parity';
+import type { TranslationMap } from './types';
+
+describe('resolveKey', () => {
+  const translations: TranslationMap = {
+    'pt-PT': { greeting: 'Olá {{name}}' },
+    'en-GB': { greeting: 'Hello {{name}}', onlyInEnglish: 'English only' },
+  };
+
+  test('resolves from the active locale', () => {
+    expect(resolveKey(translations, 'pt-PT', 'greeting', 'en-GB')).toBe('Olá {{name}}');
+  });
+
+  test('falls back to the fallback locale on a miss', () => {
+    expect(resolveKey(translations, 'pt-PT', 'onlyInEnglish', 'en-GB')).toBe('English only');
+  });
+
+  test('renders the raw key when even the fallback is missing', () => {
+    expect(resolveKey(translations, 'pt-PT', 'nowhere', 'en-GB')).toBe('nowhere');
+  });
+
+  test('never walks a chain between two non-fallback locales', () => {
+    const withThird: TranslationMap = { ...translations, 'pt-BR': {} };
+    // pt-BR has no key of its own and must NOT inherit pt-PT's — only the fallback locale counts.
+    expect(resolveKey(withThird, 'pt-BR', 'greeting', 'en-GB')).toBe('Hello {{name}}');
+  });
+
+  test('selects a plural branch via Intl.PluralRules', () => {
+    const plurals: TranslationMap = {
+      'en-GB': { files: { one: '{{count}} file', other: '{{count}} files' } },
+    };
+    expect(resolveKey(plurals, 'en-GB', 'files', 'en-GB', 1)).toBe('{{count}} file');
+    expect(resolveKey(plurals, 'en-GB', 'files', 'en-GB', 2)).toBe('{{count}} files');
+    expect(resolveKey(plurals, 'en-GB', 'files', 'en-GB', 0)).toBe('{{count}} files');
+  });
+});
+
+describe('interpolate', () => {
+  test('substitutes params', () => {
+    expect(interpolate('Hello {{name}}', { name: 'Ana' })).toBe('Hello Ana');
+  });
+
+  test('leaves an unknown placeholder untouched', () => {
+    expect(interpolate('Hello {{name}}', {})).toBe('Hello {{name}}');
+  });
+
+  test('passes a plain string through with no params', () => {
+    expect(interpolate('Hello')).toBe('Hello');
+  });
+});
+
+describe('isPluralForms', () => {
+  test('accepts an open record of CLDR categories', () => {
+    expect(isPluralForms({ one: 'a', other: 'b' })).toBe(true);
+    expect(isPluralForms({ other: 'b' })).toBe(true);
+  });
+
+  test('rejects a plain nested catalog node', () => {
+    expect(isPluralForms({ title: 'a', description: 'b' })).toBe(false);
+  });
+
+  test('rejects non-objects', () => {
+    expect(isPluralForms('a string')).toBe(false);
+    expect(isPluralForms(null)).toBe(false);
+    expect(isPluralForms(['one', 'other'])).toBe(false);
+  });
+});
+
+describe('localeDirection', () => {
+  test('ltr for English and Portuguese', () => {
+    expect(localeDirection('en-GB')).toBe('ltr');
+    expect(localeDirection('pt-PT')).toBe('ltr');
+  });
+
+  test('rtl for Arabic and Hebrew, by language subtag', () => {
+    expect(localeDirection('ar')).toBe('rtl');
+    expect(localeDirection('ar-EG')).toBe('rtl');
+    expect(localeDirection('he-IL')).toBe('rtl');
+  });
+});
+
+describe('findMissingKeys', () => {
+  test('flags a key en has and a locale lacks', () => {
+    const en = { common: { save: 'Save', cancel: 'Cancel' } };
+    const ptPT = { common: { save: 'Guardar' } };
+    expect(findMissingKeys(en, { 'pt-PT': ptPT })).toEqual([{ locale: 'pt-PT', key: 'common.cancel' }]);
+  });
+
+  test('a complete catalog has no gaps', () => {
+    const en = { common: { save: 'Save' } };
+    const ptPT = { common: { save: 'Guardar' } };
+    expect(findMissingKeys(en, { 'pt-PT': ptPT })).toEqual([]);
+  });
+
+  test('a plural-forms leaf counts as one key, not one per category', () => {
+    const en = { files: { one: '{{count}} file', other: '{{count}} files' } };
+    const ptPT = { files: { one: '{{count}} ficheiro', other: '{{count}} ficheiros' } };
+    expect(findMissingKeys(en, { 'pt-PT': ptPT })).toEqual([]);
+  });
+});
diff --git a/src/workspaces/i18n/src/resolve.ts b/src/workspaces/i18n/src/resolve.ts
index e344c1bd..dde151f7 100644
Binary files a/src/workspaces/i18n/src/resolve.ts and b/src/workspaces/i18n/src/resolve.ts differ
diff --git a/src/workspaces/i18n/src/types.ts b/src/workspaces/i18n/src/types.ts
index a3636400..4d4626fe 100644
--- a/src/workspaces/i18n/src/types.ts
+++ b/src/workspaces/i18n/src/types.ts
@@ -2,6 +2,12 @@ export type TranslationMap = Record<string, Record<string, unknown>>;
 
 export type InterpolationParams = Record<string, string | number>;
 
+/**
+ * A pluralizable catalog entry — CLDR's category set (Intl.PluralRules), never a fixed
+ * `{one, other}` shape, because some locales need up to all six (see docs/localization.md D7).
+ */
+export type PluralForms = Partial<Record<Intl.LDMLPluralRule, string>>;
+
 export type I18nContextValue = {
   t: (key: string, params?: InterpolationParams) => string;
   locale: string;
diff --git a/src/workspaces/i18n/src/useFormat.ts b/src/workspaces/i18n/src/useFormat.ts
new file mode 100644
index 00000000..d260a379
--- /dev/null
+++ b/src/workspaces/i18n/src/useFormat.ts
@@ -0,0 +1,32 @@
+import { useContext, useMemo } from 'react';
+import { I18nContext } from './I18nProvider';
+
+type DateInput = Date | number | string;
+
+/**
+ * Date, time and number formatting bound to the active locale, via `Intl` (D7) — the replacement for
+ * the ~23 `toLocale*()` call sites that otherwise follow the browser's locale and quietly contradict
+ * the rest of the UI. Moving those call sites is phase 1-3 work; this is just the hook they move to.
+ */
+export const useFormat = () => {
+  const { locale } = useContext(I18nContext);
+
+  return useMemo(() => {
+    const date = (value: DateInput, options?: Intl.DateTimeFormatOptions) =>
+      new Intl.DateTimeFormat(locale, options).format(new Date(value));
+
+    const time = (value: DateInput, options?: Intl.DateTimeFormatOptions) =>
+      new Intl.DateTimeFormat(locale, { timeStyle: 'short', ...options }).format(new Date(value));
+
+    const dateTime = (value: DateInput, options?: Intl.DateTimeFormatOptions) =>
+      new Intl.DateTimeFormat(locale, { dateStyle: 'medium', timeStyle: 'short', ...options }).format(new Date(value));
+
+    const number = (value: number, options?: Intl.NumberFormatOptions) =>
+      new Intl.NumberFormat(locale, options).format(value);
+
+    const relative = (value: number, unit: Intl.RelativeTimeFormatUnit, options?: Intl.RelativeTimeFormatOptions) =>
+      new Intl.RelativeTimeFormat(locale, options).format(value, unit);
+
+    return { date, time, dateTime, number, relative };
+  }, [locale]);
+};