next-tonguei18n, compiled. For the Next.js 16 App Router.

01 · The problem

Next.js 16 changed the rules.

Next.js 16 made Cache Components and Partial Prerendering the way to build fast apps, and renamed middleware to proxy.ts. Existing i18n libraries were designed before that, and it shows.

A Next.js 16–only i18n library that compiles ICU messages ahead of time and works with Turbopack, the default bundler. Client components ship only the messages they actually call. The locale comes from route params, never from headers, so translated pages stay in the static shell under Cache Components.

02 · How it works

A config wrapper, codegen and a per-file loader.

Design principles

1. Build time over run time. If it can be parsed, validated, typed or tree-shaken at build time, it is. ADR 0001
2. The locale is a route param. Only proxy.ts touches headers and cookies, so every translated scope can be cached and prerendered. ADR 0002
3. Standards, not lock-in. ICU MessageFormat, plain JSON files, any OpenAI-compatible model, MIT license.
4. Next.js-native, nothing else. Next.js 16+ App Router only, latest minor supported, with a nightly canary run. ADR 0006
5. Zero dependencies. Nothing beyond the next and react peers, at runtime, build time or in the CLI. ADR 0012

03 · Features planned

Everything in the README, nothing more.

04 · Quick start planned API

From install to typed output.

// tongue.config.ts
import { defineConfig } from 'next-tongue/config';

export default defineConfig({
  locales: ['en', 'de'],
  defaultLocale: 'en',
  messages: './messages/{locale}.json',
});

// next.config.ts
import { withTongue } from 'next-tongue/next'; // config wrapper: runs codegen, registers the call-site loader
export default withTongue({ cacheComponents: true });

// proxy.ts
import { createProxy } from 'next-tongue/proxy';
export const proxy = createProxy();
export const config = { matcher: ['/((?!api|_next|.*\\..*).*)'] };
// Note: this matcher skips every path containing a dot (e.g. /de/blog/v1.2). See Routing & proxy.

// messages/en.json
{ "home": { "title": "Welcome, {name}!", "inbox": "{count, plural, =0 {No messages} one {# message} other {# messages}}" } }

// app/[locale]/page.tsx
import { getT } from 'next-tongue/server';

export default async function Page({ params }: PageProps<'/[locale]'>) {
  const { locale } = await params;
  const t = await getT(locale, 'home');
  return <h1>{t('title', { name: 'Ada' })}</h1>; // t('titel') or a missing `name` is a type error
}

// app/[locale]/inbox-badge.tsx
'use client';
import { useT } from 'next-tongue/client';

export function InboxBadge({ count }: { count: number }) {
  const t = useT('home');
  return <span>{t('inbox', { count })}</span>;
}

Full walkthrough: Getting started · API reference · CLI

05 · Configuration

tongue.config.ts reference

Abridged. Full reference in docs/configuration.md.

07 · Roadmap

Where it's going.

1. v0.1

   Prove the differentiator: own ICU parser + codegen, types, getT/useT, call-site loader, Cache Components example, published bundle benchmark vs next-intl (precompile) and Paraglide — go/no-go gate

   next up
2. v0.2

   proxy.ts (always/as-needed), navigation, metadata, static export, server formatMessage, pseudo-localisation, Intl helpers, security hardening

3. v0.3

   CLI check/extract/init, dev overlay, LocaleSwitcher, translated validation errors, OG images and emails

4. v0.4

   Localized pathnames, domains, prefix: 'never', monorepo, build-time DB/CMS sources, XLIFF/CSV, translator context, stale detection, a11y/bidi checks, tongue report

5. v0.5

   tongue translate (BYO LLM, glossary, lockfile)

6. v0.9

   Beta: API freeze, root-params decision

7. v1.0

   Stable, semver guarantees

8. v1.1

   VS Code extension, per-user preferred language helper

9. post-1.0

   next-intl codemod

Scope freeze: no new features before v0.1 ships. Acceptance criteria in ROADMAP.md.