Node.js & Express Localization with i18next (Step-by-Step)
Complete 2026 guide to localizing a Node.js/Express app with i18next. Covers why server-side i18n breaks under concurrency and how per-request instances fix it, i18next-fs-backend and i18next-http-middleware setup, detection order plus a custom detector for the logged-in user, req.t in routes and res.locals.t in views, namespaces and lazy loading, the escapeValue trap for JSON vs HTML, built-in Intl formatters, CLDR plurals and context keys, locale-prefixed routes and the Vary header, localized emails from queue workers with getFixedT, error codes translated at the boundary, i18next-parser extraction with a --fail-on-update CI gate, TypeScript-typed keys, supertest coverage, and a JSON ↔ PO round trip for translators.