feat(THU-824): localize transactional email and let users pick a language

[raivieiraadriano92]

Localizes the four transactional emails (magic-link, waitlist-joined, waitlist-not-ready, waitlist-reminder) and adds a language picker to the waitlist screen so users can choose before they ever have an account.

Locale comes from X-App-Language, not a persisted column. The ticket asked for user.locale, but no such column exists and no async send path does either — every email goes out inside a request from the recipient's own client. Three of the four go to people with no user row at all, so a column would be NULL for exactly those recipients. The header is already sent by every client, and shared/i18n/locales.ts was written in anticipation of the backend reading it.

No Lingui macros on the backend. Bun has no Babel pass, so a macro import resolves to Lingui's stub and throws when called — and shouldSkipEmail() hides that in dev and test, so it would surface first in production on the sign-in path. Backend copy uses the plain i18n._({ id }) runtime instead, which extracts identically, and a lint rule bans the macro specifiers.

Catalogs. backend/lingui.config.ts owns a second, small .po catalog compiled to committed .ts (Bun has no .po loader). Same gettext format and same locale set as the frontend, so translators see one pipeline. All 14 strings are translated in de/fr/es/pt-BR/ja.

Testing

bun run test:backend · bun run test · bun run i18n:check. Emails were sent end-to-end through Resend in every locale.

To see a localized email: send X-App-Language: <tag> to POST /v1/waitlist/join. Note the pseudo-locale is deliberately refused over the wire — it's reachable only via getEmailI18n('en-XA') in the preview and tests.

Notes for review

• The translations are mine, not a native speaker's. Worth an l10n review of the German and Japanese paragraphs before this reaches users, and Pontoon (THU-827) will see them as existing strings to refine rather than blanks to fill.
• Not fixed here: the 429 in waitlist/routes.ts sends English message text that the client renders verbatim, so a German user can still get a German email and an English toast from one click. The clean fix is a frontend mapping for code_already_sent, not localizing backend prose.

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Semgrep Security Scan

No security issues found.

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PR Metrics

Metric	Value
Lines changed (prod code)	+1045 / -88
JS bundle size (gzipped)	🟢 638.2 KB → 638.9 KB (+732 B, +0.1%)
Test coverage	🟢 81.81% → 81.78% (+-0.0%)
Performance (preview)	Preview not ready — Render deploy may have timed out
Accessibility	—
Best Practices	—
SEO	—

Updated Wed, 02 Sep 2026 17:33:56 GMT · run #2890

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Preview environment destroyed 🧹

Stack preview-pr-1255 and its Cloudflare subdomain have been cleaned up.

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