Daeseon Yoo
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·Tech retro·2 min

i18n: next-intl with path-based routing across 5 locales

Migrated the entire frontend to `next-intl` with path-based routing (`/en`, `/ko`, `/ja`, `/zh`, `/es`), default English. All 17 user-facing files were keyed; `<Link>` and `useRouter` imports across the app moved to `@/i18n/navigation` wrappers. ja/zh/es fall back to English copy for now (translation is a non-engineering task).

The decision to add UI internationalization was a product call, not a tech one: the user-facing default switched to English (the actual learner audience), with Korean as a hand-translated fallback. ja/zh/es ship as English fallback until I (or contributors) translate them — the routing is ready.

Stack: next-intl with path-based routing. URLs become /en/library, /ko/library, etc. The default locale (en) is not implicit — every URL carries a prefix. This is the variant that survives prerendering and SEO without surprises.

The migration was almost entirely mechanical:

  1. Routing — i18n/routing.ts (locale list + default), i18n/request.ts (load messages by locale), i18n/navigation.ts (typed <Link> and useRouter wrappers), middleware.ts (createMiddleware(routing)), next.config.ts plugin.
  2. App directory move — app/* → app/[locale]/*. Root layout.tsx deleted; the locale-aware layout lives at app/[locale]/layout.tsx and is the canonical root.
  3. Imports — every from "next/link" and the useRouter/usePathname from "next/navigation" had to switch to "@/i18n/navigation". Did it with sed -i '' across 14 files. The trap was that next/link is a default export while next-intl/navigation's Link is named — a second sed to fix import Link from → import { Link } from.
  4. Keys — pulled every Korean string into messages/{en,ko,...}.json with structured namespaces (common, home, nav, auth, library, import, review, settings, discover, video, clipCreate, clipPlayer, note, recording, transcript, streak, shortcuts, globalError, notFound, analysis, blind, audioMode, locale).
  5. Verification — added e2e/tests/i18n.spec.ts covering 5 locale routes + locale selector flips /en → /ko and back.

Lesson: centralize navigation imports from day one if you might ever localize. Replacing them later is a one-shot sed either way, but the named/default import disagreement between next/link and next-intl/navigation cost an extra round.

The locale selector lives in the app header and writes the selected locale to localStorage via the wrapped router; cookie-based detection happens via middleware on first request.

Commit: ba90e00 — [feat] i18n + 직독직해 + Output quizzes + Decks + Playlist + project log