Internationalization Overview

Internationalization (often abbreviated i18n — "i", 18 letters, "n") is the practice of building an application so it can serve content in more than one language and adapt to region-specific conventions like date formats, number formats, and currencies. In a Next.js app this usually touches three things at once: which URL a visitor lands on for a given locale, where translated strings live, and how the right ones get picked at render time.

The App Router has no built-in i18n routing

This is the detail that trips people up coming from the older Pages Router: Pages Router had i18n routing as a first-class, next.config.js-configured feature (i18n.locales, automatic locale detection, automatic /en//de prefixing). The App Router deliberately dropped that built-in system. Locale-aware routing in the App Router is instead something you build yourself, typically with a [locale] dynamic segment and middleware — or, in practice, with the help of a library that implements this pattern for you.

Note

This isn't a regression — it's a deliberate trade for flexibility. Baking locale routing into the framework meant one opinionated shape for everyone; leaving it to the app (or a library on top of the App Router's primitives) means it composes with whatever routing structure your app already has.

Popular libraries

Library	Notes
next-intl	Built specifically for the App Router; provides `[locale]` routing helpers, a `useTranslations` hook, and Server Component support
next-i18next	Mature and widely used, but designed around the Pages Router — needs workarounds for full App Router support
react-intl (FormatJS)	Framework-agnostic i18n primitives (message formatting, pluralization, dates) — often paired with a routing solution rather than providing one itself

This site's own routing (the [locale] segment you can see in the URL right now) uses next-intl, which is also the most commonly recommended choice for new App Router projects as of this writing.

i18n covers both routing (which URL serves which language) and content (translated strings, formatted dates/numbers).
Unlike the Pages Router, the App Router has no built-in locale routing system — it is built on top of App Router primitives.
next-intl is the most common choice for new App Router projects; next-i18next targets the Pages Router; react-intl focuses on formatting rather than routing.
The next two pages dig into the two halves of the problem: locale-aware routing, and managing the translations themselves.