diff --git a/services/platform-service/src/lib/cosmos-init.ts b/services/platform-service/src/lib/cosmos-init.ts
index a49377bf..486ac003 100644
--- a/services/platform-service/src/lib/cosmos-init.ts
+++ b/services/platform-service/src/lib/cosmos-init.ts
@@ -182,6 +182,9 @@ const CONTAINER_DEFS: Record<string, ContainerConfig> = {
   // API Versioning (P3)
   api_versions: { partitionKeyPath: '/productId' },
   api_version_pins: { partitionKeyPath: '/productId' },
+  // i18n (P3)
+  translations: { partitionKeyPath: '/locale' },
+  i18n_locales: { partitionKeyPath: '/locale' },
 };
 
 export async function initCosmosIfNeeded(): Promise<void> {
diff --git a/services/platform-service/src/modules/i18n/i18n.test.ts b/services/platform-service/src/modules/i18n/i18n.test.ts
new file mode 100644
index 00000000..170ceb8b
--- /dev/null
+++ b/services/platform-service/src/modules/i18n/i18n.test.ts
@@ -0,0 +1,130 @@
+/**
+ * i18n module — unit tests.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { UpsertTranslationSchema, BulkImportSchema, CreateLocaleSchema } from './types.js';
+
+// ── UpsertTranslationSchema ──────────────────────────────────
+
+describe('UpsertTranslationSchema', () => {
+  it('accepts valid value', () => {
+    const result = UpsertTranslationSchema.parse({ value: 'Save' });
+    expect(result.value).toBe('Save');
+  });
+
+  it('rejects empty value', () => {
+    expect(() => UpsertTranslationSchema.parse({ value: '' })).toThrow();
+  });
+
+  it('rejects value over 5000 chars', () => {
+    expect(() => UpsertTranslationSchema.parse({ value: 'x'.repeat(5001) })).toThrow();
+  });
+});
+
+// ── BulkImportSchema ─────────────────────────────────────────
+
+describe('BulkImportSchema', () => {
+  it('accepts valid translations map', () => {
+    const result = BulkImportSchema.parse({
+      translations: { 'common.save': 'Save', 'common.cancel': 'Cancel' },
+    });
+    expect(Object.keys(result.translations)).toHaveLength(2);
+    expect(result.overwrite).toBe(false);
+  });
+
+  it('accepts overwrite flag', () => {
+    const result = BulkImportSchema.parse({
+      translations: { 'common.save': 'Sauvegarder' },
+      overwrite: true,
+    });
+    expect(result.overwrite).toBe(true);
+  });
+
+  it('rejects empty value in translations', () => {
+    expect(() => BulkImportSchema.parse({ translations: { 'common.save': '' } })).toThrow();
+  });
+
+  it('rejects value over 5000 chars in translations', () => {
+    expect(() =>
+      BulkImportSchema.parse({ translations: { 'common.save': 'x'.repeat(5001) } })
+    ).toThrow();
+  });
+});
+
+// ── CreateLocaleSchema ───────────────────────────────────────
+
+describe('CreateLocaleSchema', () => {
+  it('accepts valid locale', () => {
+    const result = CreateLocaleSchema.parse({
+      locale: 'fr',
+      name: 'French',
+      nativeName: 'Français',
+    });
+    expect(result.locale).toBe('fr');
+    expect(result.direction).toBe('ltr');
+    expect(result.enabled).toBe(true);
+  });
+
+  it('accepts locale with region', () => {
+    const result = CreateLocaleSchema.parse({
+      locale: 'fr-CA',
+      name: 'French (Canada)',
+      nativeName: 'Français (Canada)',
+    });
+    expect(result.locale).toBe('fr-CA');
+  });
+
+  it('accepts RTL direction', () => {
+    const result = CreateLocaleSchema.parse({
+      locale: 'ar',
+      name: 'Arabic',
+      nativeName: 'العربية',
+      direction: 'rtl',
+    });
+    expect(result.direction).toBe('rtl');
+  });
+
+  it('rejects invalid locale format', () => {
+    expect(() =>
+      CreateLocaleSchema.parse({ locale: 'INVALID', name: 'Test', nativeName: 'Test' })
+    ).toThrow();
+  });
+
+  it('rejects single-char locale', () => {
+    expect(() =>
+      CreateLocaleSchema.parse({ locale: 'f', name: 'Test', nativeName: 'Test' })
+    ).toThrow();
+  });
+
+  it('rejects empty name', () => {
+    expect(() =>
+      CreateLocaleSchema.parse({ locale: 'fr', name: '', nativeName: 'Français' })
+    ).toThrow();
+  });
+
+  it('rejects empty nativeName', () => {
+    expect(() =>
+      CreateLocaleSchema.parse({ locale: 'fr', name: 'French', nativeName: '' })
+    ).toThrow();
+  });
+
+  it('defaults enabled to true', () => {
+    const result = CreateLocaleSchema.parse({
+      locale: 'de',
+      name: 'German',
+      nativeName: 'Deutsch',
+    });
+    expect(result.enabled).toBe(true);
+  });
+
+  it('accepts disabled locale', () => {
+    const result = CreateLocaleSchema.parse({
+      locale: 'de',
+      name: 'German',
+      nativeName: 'Deutsch',
+      enabled: false,
+    });
+    expect(result.enabled).toBe(false);
+  });
+});
diff --git a/services/platform-service/src/modules/i18n/repository.ts b/services/platform-service/src/modules/i18n/repository.ts
new file mode 100644
index 00000000..98847a1f
--- /dev/null
+++ b/services/platform-service/src/modules/i18n/repository.ts
@@ -0,0 +1,211 @@
+/**
+ * i18n repository — Cosmos DB CRUD for translations and locales.
+ * Fallback chain: requested locale → language-only → 'en'.
+ */
+
+import { getRegisteredContainer } from '@bytelyst/cosmos';
+import type { TranslationDoc, LocaleDoc, BulkImportInput, CreateLocaleInput } from './types.js';
+
+// ── Container Access ──────────────────────────────────────────
+
+function getTranslationsContainer() {
+  return getRegisteredContainer('translations');
+}
+
+function getLocalesContainer() {
+  return getRegisteredContainer('i18n_locales');
+}
+
+// ── Locale CRUD ──────────────────────────────────────────────
+
+export async function listLocales(productId: string): Promise<LocaleDoc[]> {
+  const { resources } = await getLocalesContainer()
+    .items.query<LocaleDoc>({
+      query: 'SELECT * FROM c WHERE c.productId = @pid ORDER BY c.locale ASC',
+      parameters: [{ name: '@pid', value: productId }],
+    })
+    .fetchAll();
+  return resources;
+}
+
+export async function getLocale(locale: string): Promise<LocaleDoc | null> {
+  try {
+    const { resource } = await getLocalesContainer().item(locale, locale).read<LocaleDoc>();
+    return resource ?? null;
+  } catch {
+    return null;
+  }
+}
+
+export async function createLocale(
+  productId: string,
+  input: CreateLocaleInput
+): Promise<LocaleDoc> {
+  const now = new Date().toISOString();
+  const doc: LocaleDoc = {
+    id: input.locale,
+    locale: input.locale,
+    name: input.name,
+    nativeName: input.nativeName,
+    direction: input.direction ?? 'ltr',
+    productId,
+    enabled: input.enabled ?? true,
+    createdAt: now,
+    updatedAt: now,
+  };
+  await getLocalesContainer().items.create(doc);
+  return doc;
+}
+
+// ── Translation CRUD ─────────────────────────────────────────
+
+function translationId(locale: string, key: string): string {
+  return `${locale}:${key}`;
+}
+
+function extractNamespace(key: string): string {
+  const idx = key.indexOf('.');
+  return idx > 0 ? key.substring(0, idx) : key;
+}
+
+export async function getTranslations(
+  locale: string,
+  productId: string,
+  namespace?: string
+): Promise<Record<string, string>> {
+  const conditions = ['c.locale = @loc', 'c.productId = @pid'];
+  const params = [
+    { name: '@loc', value: locale },
+    { name: '@pid', value: productId },
+  ];
+
+  if (namespace) {
+    conditions.push('c.namespace = @ns');
+    params.push({ name: '@ns', value: namespace });
+  }
+
+  const { resources } = await getTranslationsContainer()
+    .items.query<TranslationDoc>({
+      query: `SELECT * FROM c WHERE ${conditions.join(' AND ')}`,
+      parameters: params,
+    })
+    .fetchAll();
+
+  const result: Record<string, string> = {};
+  for (const doc of resources) {
+    result[doc.key] = doc.value;
+  }
+  return result;
+}
+
+export async function getTranslationsWithFallback(
+  locale: string,
+  productId: string,
+  namespace?: string
+): Promise<{ translations: Record<string, string>; resolvedLocale: string }> {
+  // Try exact locale (e.g. 'fr-CA')
+  let translations = await getTranslations(locale, productId, namespace);
+  if (Object.keys(translations).length > 0) {
+    return { translations, resolvedLocale: locale };
+  }
+
+  // Try language-only (e.g. 'fr')
+  const langOnly = locale.split('-')[0];
+  if (langOnly !== locale) {
+    translations = await getTranslations(langOnly, productId, namespace);
+    if (Object.keys(translations).length > 0) {
+      return { translations, resolvedLocale: langOnly };
+    }
+  }
+
+  // Fallback to 'en'
+  if (langOnly !== 'en') {
+    translations = await getTranslations('en', productId, namespace);
+    return { translations, resolvedLocale: 'en' };
+  }
+
+  return { translations: {}, resolvedLocale: locale };
+}
+
+export async function upsertTranslation(
+  locale: string,
+  key: string,
+  value: string,
+  productId: string
+): Promise<TranslationDoc> {
+  const id = translationId(locale, key);
+  const namespace = extractNamespace(key);
+  const now = new Date().toISOString();
+
+  // Try to read existing
+  try {
+    const { resource } = await getTranslationsContainer().item(id, locale).read<TranslationDoc>();
+    if (resource) {
+      const updated: TranslationDoc = {
+        ...resource,
+        value,
+        updatedAt: now,
+      };
+      await getTranslationsContainer().item(id, locale).replace(updated);
+      return updated;
+    }
+  } catch {
+    // Does not exist — create
+  }
+
+  const doc: TranslationDoc = {
+    id,
+    locale,
+    key,
+    namespace,
+    value,
+    productId,
+    createdAt: now,
+    updatedAt: now,
+  };
+  await getTranslationsContainer().items.create(doc);
+  return doc;
+}
+
+export async function bulkImport(
+  locale: string,
+  productId: string,
+  input: BulkImportInput
+): Promise<{ imported: number; skipped: number }> {
+  let imported = 0;
+  let skipped = 0;
+
+  for (const [key, value] of Object.entries(input.translations)) {
+    const id = translationId(locale, key);
+
+    if (!input.overwrite) {
+      // Check if already exists
+      try {
+        const { resource } = await getTranslationsContainer()
+          .item(id, locale)
+          .read<TranslationDoc>();
+        if (resource) {
+          skipped++;
+          continue;
+        }
+      } catch {
+        // Does not exist — proceed with create
+      }
+    }
+
+    await upsertTranslation(locale, key, value, productId);
+    imported++;
+  }
+
+  return { imported, skipped };
+}
+
+export async function deleteTranslation(locale: string, key: string): Promise<boolean> {
+  const id = translationId(locale, key);
+  try {
+    await getTranslationsContainer().item(id, locale).delete();
+    return true;
+  } catch {
+    return false;
+  }
+}
diff --git a/services/platform-service/src/modules/i18n/routes.ts b/services/platform-service/src/modules/i18n/routes.ts
new file mode 100644
index 00000000..b5888be4
--- /dev/null
+++ b/services/platform-service/src/modules/i18n/routes.ts
@@ -0,0 +1,84 @@
+/**
+ * i18n routes.
+ * Public: get translations, list locales. Admin: upsert, bulk import, manage locales.
+ */
+
+import type { FastifyInstance } from 'fastify';
+import { UnauthorizedError, ForbiddenError, NotFoundError } from '../../lib/errors.js';
+import { getRequestProductId, getRequestProductIdForPublic } from '../../lib/request-context.js';
+import { UpsertTranslationSchema, BulkImportSchema, CreateLocaleSchema } from './types.js';
+import {
+  listLocales,
+  getTranslationsWithFallback,
+  upsertTranslation,
+  bulkImport,
+  deleteTranslation,
+  createLocale,
+} from './repository.js';
+
+function requireAdmin(req: { jwtPayload?: { sub: string; role?: string } }): string {
+  if (!req.jwtPayload?.sub) throw new UnauthorizedError('Authentication required');
+  if (req.jwtPayload.role !== 'admin') throw new ForbiddenError('Admin access required');
+  return req.jwtPayload.sub;
+}
+
+export async function i18nRoutes(app: FastifyInstance): Promise<void> {
+  // ── Public: List available locales ────────────────────────
+  app.get('/i18n/locales', async req => {
+    const productId = getRequestProductIdForPublic(req);
+    const locales = await listLocales(productId);
+    return locales.filter(l => l.enabled);
+  });
+
+  // ── Public: Get translations for locale ──────────────────
+  app.get<{ Params: { locale: string } }>('/i18n/:locale', async req => {
+    const productId = getRequestProductIdForPublic(req);
+    return getTranslationsWithFallback(req.params.locale, productId);
+  });
+
+  // ── Public: Get translations for locale + namespace ──────
+  app.get<{ Params: { locale: string; namespace: string } }>(
+    '/i18n/:locale/:namespace',
+    async req => {
+      const productId = getRequestProductIdForPublic(req);
+      return getTranslationsWithFallback(req.params.locale, productId, req.params.namespace);
+    }
+  );
+
+  // ── Admin: Upsert translation ────────────────────────────
+  app.put<{ Params: { locale: string; key: string } }>('/admin/i18n/:locale/:key', async req => {
+    requireAdmin(req);
+    const productId = getRequestProductId(req);
+    const { value } = UpsertTranslationSchema.parse(req.body);
+    return upsertTranslation(req.params.locale, req.params.key, value, productId);
+  });
+
+  // ── Admin: Bulk import translations ──────────────────────
+  app.post<{ Params: { locale: string } }>('/admin/i18n/:locale/import', async req => {
+    requireAdmin(req);
+    const productId = getRequestProductId(req);
+    const input = BulkImportSchema.parse(req.body);
+    return bulkImport(req.params.locale, productId, input);
+  });
+
+  // ── Admin: Delete translation ────────────────────────────
+  app.delete<{ Params: { locale: string; key: string } }>(
+    '/admin/i18n/:locale/:key',
+    async (req, reply) => {
+      requireAdmin(req);
+      const ok = await deleteTranslation(req.params.locale, req.params.key);
+      if (!ok) throw new NotFoundError('Translation not found');
+      reply.status(204);
+    }
+  );
+
+  // ── Admin: Create locale ─────────────────────────────────
+  app.post('/admin/i18n/locales', async (req, reply) => {
+    requireAdmin(req);
+    const productId = getRequestProductId(req);
+    const input = CreateLocaleSchema.parse(req.body);
+    const locale = await createLocale(productId, input);
+    reply.status(201);
+    return locale;
+  });
+}
diff --git a/services/platform-service/src/modules/i18n/types.ts b/services/platform-service/src/modules/i18n/types.ts
new file mode 100644
index 00000000..5ac494c8
--- /dev/null
+++ b/services/platform-service/src/modules/i18n/types.ts
@@ -0,0 +1,56 @@
+/**
+ * i18n module — types and schemas.
+ * Translation management with locale/namespace structure and fallback chain.
+ */
+
+import { z } from 'zod';
+
+export interface TranslationDoc {
+  id: string; // `${locale}:${key}`
+  locale: string; // partition key — e.g. 'en', 'fr', 'ja'
+  key: string; // dot-separated: 'common.save', 'dashboard.title'
+  namespace: string; // first segment of key: 'common', 'dashboard'
+  value: string;
+  productId: string;
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface LocaleDoc {
+  id: string; // locale code
+  locale: string; // partition key (same as id)
+  name: string; // 'English', 'Français'
+  nativeName: string; // 'English', 'Français'
+  direction: 'ltr' | 'rtl';
+  productId: string;
+  enabled: boolean;
+  createdAt: string;
+  updatedAt: string;
+}
+
+// ── Schemas ────────────────────────────────────────────────────
+
+export const UpsertTranslationSchema = z.object({
+  value: z.string().min(1).max(5000),
+});
+
+export const BulkImportSchema = z.object({
+  translations: z.record(z.string().min(1).max(5000)), // { "common.save": "Save", ... }
+  overwrite: z.boolean().default(false),
+});
+
+export const CreateLocaleSchema = z.object({
+  locale: z
+    .string()
+    .min(2)
+    .max(10)
+    .regex(/^[a-z]{2}(-[A-Z]{2})?$/),
+  name: z.string().min(1).max(100),
+  nativeName: z.string().min(1).max(100),
+  direction: z.enum(['ltr', 'rtl']).default('ltr'),
+  enabled: z.boolean().default(true),
+});
+
+export type UpsertTranslationInput = z.infer<typeof UpsertTranslationSchema>;
+export type BulkImportInput = z.infer<typeof BulkImportSchema>;
+export type CreateLocaleInput = z.infer<typeof CreateLocaleSchema>;
diff --git a/services/platform-service/src/server.ts b/services/platform-service/src/server.ts
index d35b8231..964ebb3c 100644
--- a/services/platform-service/src/server.ts
+++ b/services/platform-service/src/server.ts
@@ -98,6 +98,7 @@ import { tenantRoutes } from './modules/tenants/routes.js';
 import { retentionRoutes } from './modules/retention/routes.js';
 import { backupRoutes } from './modules/backups/routes.js';
 import { apiVersioningRoutes } from './modules/api-versioning/routes.js';
+import { i18nRoutes } from './modules/i18n/routes.js';
 import aiDiagnosticsRoutes from './modules/ai-diagnostics/routes.js';
 import { initCosmosIfNeeded } from './lib/cosmos-init.js';
 import { config } from './lib/config.js';
@@ -264,6 +265,8 @@ await app.register(tenantRoutes, { prefix: '/api' });
 await app.register(retentionRoutes, { prefix: '/api' });
 await app.register(backupRoutes, { prefix: '/api' });
 await app.register(apiVersioningRoutes, { prefix: '/api' });
+// i18n translations
+await app.register(i18nRoutes, { prefix: '/api' });
 
 // AI Diagnostics (NL query, LLM root-cause, error clustering)
 await app.register(aiDiagnosticsRoutes, { prefix: '/api/ai-diagnostics' });