🎃 Set up i18n in the frontend and translate the sidebar and sign-in form (DE, IT) #1008

Part of the October Challenge #997.

Why

The website guides and the README already exist in German and Italian (content/guides/de, content/guides/it, README.de.md), but the admin UI is English only. Every string is hard-coded in JSX, <html lang="en"> is fixed (app/layout.tsx:72), and there is no i18n library. The only translated UI is the cookie banner (components/cookie-consent.tsx:37-391, browser language detection at :424-426), which only renders when GTM is configured. Teams whose staff work in German or Italian have to put up with an English admin UI, and adding a language later gets harder with every new page.

This issue lays the foundation and proves it on two surfaces: the sidebar and the sign-in form. Other pages follow in separate issues, one area at a time.

Self-hosted vs Cloud

• Self-hosted: same in Community and Business. Frontend only: no backend or database change, no ee/ code.
• Cloud (DEPLOYMENT_MODE=cloud): same image, same behaviour. The language is a per-browser preference stored in a cookie, not a workspace or instance setting, so members of one workspace can use different languages.
• Who can use it: everyone, including signed-out visitors on /login.

What to build

1. Library: next-intl in its "without i18n routing" setup. Reasons: it supports the App Router with server and client components, ICU messages (plurals such as "1 tool" / "3 tools") and typed message keys. Without routing there are no /de/... URL prefixes, so the auth gate in proxy.ts, the backend rewrites in next.config.ts and every ?redirect= link keep working unchanged. Almost every page is a client component, so the messages are passed once through NextIntlClientProvider in the root layout. Before you start, check that the current next-intl release supports the next version in packages/frontend/package.json (16.x) and name the version in the PR. This Next.js version has breaking changes (see packages/frontend/AGENTS.md), so read the docs it ships in node_modules/next/dist/docs/ as well. If next-intl does not fit, comment here before building an alternative.
2. Locale resolution (server side): the cookie amcp_locale if it holds a supported locale, otherwise the best match from Accept-Language, otherwise en. Supported: en, de, it. Set <html lang> from it in app/layout.tsx:72. The root layout is already force-dynamic (:58), so reading the cookie per request costs nothing extra. app/global-error.tsx:30 can stay en.
3. Messages: packages/frontend/messages/{en,de,it}.json, nested by area (nav.*, signIn.*). English is the source. A key missing in de or it falls back to English and never shows a raw key.
4. Sidebar (components/app-sidebar.tsx): the group titles and item labels in NAV (:45-73), "Switch workspace" (:156), "Log out" (:188), "Settings" (:205), the "Workspace" fallback (:216) and the "Star on GitHub" texts (components/github-star-button.tsx:54-60). hrefs and the active-item logic stay as they are.
5. Sign-in form (app/login/page.tsx:851-1144, sign-in mode only, !isRegister): heading, labels, placeholders, the submit button states, "Forgot password?", the recovery-code texts, the "or" divider, "Don't have an account? Register", "Email verified successfully!…", "Signing you in…", and the client-side messages shown in this mode. The registration and Cloud sign-up variants and the verification and licence steps stay English in this issue.
6. Language switcher: a select (English, Deutsch, Italiano) in Settings → Profile (app/settings/page.tsx) and a small one under the sign-in card. It writes the cookie (one year, SameSite=Lax, Path=/) and calls router.refresh().
7. Translations: match the tone of the existing German and Italian guides in content/guides/de and content/guides/it. If you are not a native speaker of one of the languages, say so in the PR so a maintainer can review those strings.
8. Contributor note: a short "Translating the UI" section in .github/CONTRIBUTING.md: where the messages live, how to add a key, English is the source.

Where to look

• packages/frontend/src/app/layout.tsx:51-89: root layout (force-dynamic, <html lang>).
• packages/frontend/src/app/providers.tsx: client providers to wrap.
• packages/frontend/src/proxy.ts: auth gate and public paths; must keep working unchanged.
• packages/frontend/src/components/app-sidebar.tsx:45-73, :156, :188, :205, :216.
• packages/frontend/src/app/login/page.tsx:851-1144: the auth form.
• packages/frontend/src/components/cookie-consent.tsx:37-391, :424-426: the banner's own translations (leave them as they are).
• packages/frontend/src/lib/translation-safe-dom.ts: keeps browser page translation from crashing React; keep it.
• packages/frontend/tests/e2e/: many tests match English text, so English must stay the result for Accept-Language: en-US, Playwright's default.

Acceptance criteria

• Without the cookie, Accept-Language: de-DE renders /login and the sidebar in German with <html lang="de">; it-IT gives Italian; fr-FR gives English.
• A language chosen in Settings persists across reloads and sign-out.
• Playwright tests for the cases above (test.use({ locale: 'de-DE' })), plus a key-parity test that loads the three JSON files and fails when de or it misses or adds a key. All existing tests pass unchanged (cd packages/frontend && npm run test:e2e).
• URLs are unchanged: login-redirect-query.spec.ts and login-redirect-safe.spec.ts pass.
• npm run lint, npx tsc --noEmit -p tsconfig.json and npm run build pass in packages/frontend.
• Screenshots of the sidebar and the sign-in form in German and Italian in the PR.

Out of scope

• Other pages (follow-up issues), the backend-rendered OAuth sign-in and consent pages (packages/backend/src/auth/auth-page.ts), emails and API error messages.
• Locale-prefixed URLs, right-to-left languages, languages other than German and Italian.
• Storing the language on the user account.
• Changing date or number formatting.

Size

L (several days, may be split: library, locale resolution and sidebar first; sign-in form and switcher second).

How to claim

Comment "I'd like to work on this" and we'll assign you. Rules in #997.