diff --git a/docker-compose.yml b/docker-compose.yml
index 6eedd7e..9ecb25d 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -1,5 +1,8 @@
 # ponytail: dev-server images (not multi-stage prod builds) — this is a demo.
-# `docker compose up` → app at http://localhost:4200, Swagger at http://localhost:5000/swagger
+# `docker compose up` → app at http://localhost:4200 (LOCALIZED: /nl/ + /en/, the header
+# language switcher works), Swagger at http://localhost:5000/swagger. The web container does a
+# one-time `ng build --localize` then serves both locale bundles statically (with /api proxied);
+# for a fast HMR loop use `npm start` locally instead (nl-only at /).
 services:
   api:
     image: mcr.microsoft.com/dotnet/sdk:10.0
@@ -30,12 +33,14 @@ services:
     # msgpackr-extract) ship prebuilt glibc binaries, so no build toolchain is needed.
     image: node:24-slim
     working_dir: /app
-    # Uses the committed generated client (no codegen at startup); proxies /api → api container.
-    # ponytail: `--no-fund --loglevel=error` silences npm 11's startup noise (deprecation +
-    # allow-scripts + funding) so the demo boots clean; ng serve output is unaffected. The
-    # deprecation lines are Angular 22's webpack-builder (@angular-devkit/build-angular) — a
-    # known upstream deprecation; migrating to @angular/build is a separate, larger effort.
-    command: sh -c "npm ci --no-fund --loglevel=error && npx ng serve --host 0.0.0.0 --proxy-config proxy.conf.docker.json"
+    # Uses the committed generated client (no codegen at startup). Builds BOTH locale bundles
+    # once (`ng build --localize`) then serves them statically via scripts/serve-i18n.mjs
+    # (per-locale SPA fallback + /api reverse-proxy → the api container), so the language
+    # switcher actually switches. ponytail: `--no-fund --loglevel=error` silences npm 11 noise.
+    command: sh -c "npm ci --no-fund --loglevel=error && npx ng build --localize && node scripts/serve-i18n.mjs"
+    environment:
+      - PORT=4200
+      - API_PROXY_TARGET=http://api:5000
     volumes:
       - ./:/app:z
       - web-modules:/app/node_modules
diff --git a/docs/reference/architecture/ARCHITECTURE.md b/docs/reference/architecture/ARCHITECTURE.md
index 4962a3f..aaacbdc 100644
--- a/docs/reference/architecture/ARCHITECTURE.md
+++ b/docs/reference/architecture/ARCHITECTURE.md
@@ -534,8 +534,9 @@ no longer hand-written DTOs — it's an **NSwag-generated typed client**
 - **Proxy.** The app uses a relative base URL (`apiBaseUrl: ''`), so `/api` calls are
   same-origin and `ng serve` proxies them to the backend on `:5000`.
   ([`environment.ts`](../../../src/environments/environment.ts),
-  [`proxy.conf.json`](../../../proxy.conf.json); `proxy.conf.docker.json` targets the
-  compose service.)
+  [`proxy.conf.json`](../../../proxy.conf.json)). Under `docker compose up` the app is served
+  localized (both locales) by `scripts/serve-i18n.mjs`, which proxies `/api` to the `api`
+  container itself (`API_PROXY_TARGET`).
 - **Client → HttpClient seam.** The NSwag client's `fetch` is routed through Angular's
   `HttpClient` by `httpClientFetch` — the one place cross-cutting concerns live:
   `X-Correlation-Id` on every call, `Idempotency-Key` on non-GETs, a 10 s timeout, and
diff --git a/proxy.conf.docker.json b/proxy.conf.docker.json
deleted file mode 100644
index fc9056a..0000000
--- a/proxy.conf.docker.json
+++ /dev/null
@@ -1,7 +0,0 @@
-{
-  "/api": {
-    "target": "http://api:5000",
-    "secure": false,
-    "changeOrigin": true
-  }
-}
diff --git a/scripts/serve-i18n.mjs b/scripts/serve-i18n.mjs
index ef64761..f851ca6 100644
--- a/scripts/serve-i18n.mjs
+++ b/scripts/serve-i18n.mjs
@@ -4,12 +4,15 @@
 // Plain `ng serve` (npm start) serves only nl at /, so switching 404s there — this static
 // server serves both locale subdirs with per-locale SPA fallback (a miss under /<locale>/…
 // serves that locale's index.html), so deep-link switches resolve. Demo only, not prod infra.
-import { createServer } from 'node:http';
+import { createServer, request as httpRequest } from 'node:http';
 import { readFile } from 'node:fs/promises';
 import { join, extname, normalize } from 'node:path';
 
 const ROOT = 'dist/atomic-design-poc/browser';
-const PORT = 4300;
+const PORT = Number(process.env.PORT ?? 4300);
+// Reverse-proxy /api → backend so the served app is the whole demo, not just static pages.
+// Docker sets API_PROXY_TARGET=http://api:5000; locally it defaults to the dev backend.
+const API_TARGET = new URL(process.env.API_PROXY_TARGET ?? 'http://localhost:5000');
 const MIME = {
   '.html': 'text/html',
   '.js': 'text/javascript',
@@ -28,7 +31,27 @@ const send = (res, status, body, type) => {
 };
 
 createServer(async (req, res) => {
-  const url = decodeURIComponent((req.url ?? '/').split('?')[0]);
+  const rawUrl = req.url ?? '/';
+  // Proxy API calls to the backend (keep the raw url incl. query).
+  if (rawUrl.startsWith('/api')) {
+    const proxied = httpRequest(
+      {
+        hostname: API_TARGET.hostname,
+        port: API_TARGET.port,
+        path: rawUrl,
+        method: req.method,
+        headers: req.headers,
+      },
+      (pr) => {
+        res.writeHead(pr.statusCode ?? 502, pr.headers);
+        pr.pipe(res);
+      },
+    );
+    proxied.on('error', () => send(res, 502, 'Bad gateway (API)', 'text/plain'));
+    req.pipe(proxied);
+    return;
+  }
+  const url = decodeURIComponent(rawUrl.split('?')[0]);
   // Landing at / has no locale bundle — redirect to Dutch.
   if (url === '/') {
     res.writeHead(302, { location: '/nl/' });
diff --git a/src/app/app.config.ts b/src/app/app.config.ts
index 6d0e2c2..6b55e65 100644
--- a/src/app/app.config.ts
+++ b/src/app/app.config.ts
@@ -9,6 +9,7 @@ import type { ActivatedRouteSnapshot } from '@angular/router';
 import { provideHttpClient, withInterceptors } from '@angular/common/http';
 import { registerLocaleData } from '@angular/common';
 import localeNl from '@angular/common/locales/nl';
+import localeEn from '@angular/common/locales/en';
 
 import { routes } from './app.routes';
 import { scenarioInterceptor } from '@shared/infrastructure/scenario.interceptor';
@@ -19,7 +20,9 @@ import { SessionStore } from '@auth/application/session.store';
 import { provideRouteFocus } from '@shared/layout/route-focus';
 import { provideUnloadFlush } from '@shared/application/pending-saves';
 
+// Both locales' data so DatePipe/number pipes work for whichever bundle is active.
 registerLocaleData(localeNl);
+registerLocaleData(localeEn);
 
 export const appConfig: ApplicationConfig = {
   providers: [
@@ -50,7 +53,10 @@ export const appConfig: ApplicationConfig = {
     provideHttpClient(withInterceptors(isDevMode() ? [scenarioInterceptor, roleInterceptor] : [])),
     provideApiClient(),
     { provide: SESSION_PORT, useExisting: SessionStore },
-    { provide: LOCALE_ID, useValue: 'nl' },
+    // Per-bundle locale: the localize build sets `$localize.locale` ('nl'/'en'); the
+    // non-localized dev/source build leaves it undefined → fall back to 'nl'. (Was hardcoded
+    // 'nl', which mis-formatted dates/numbers in the en bundle.)
+    { provide: LOCALE_ID, useFactory: () => $localize.locale ?? 'nl' },
     provideRouteFocus(),
     provideUnloadFlush(),
   ],