next-intl (App Router)

Setup and usage of next-intl with prefix-based locale routing (e.g. /en/about, /ja/about). Reference: this project (Tokyo Sounds).

File layout

Keep this structure:

├── messages/
│   ├── en.json
│   ├── ja.json
│   └── ...
├── next.config.ts
└── src/
    ├── i18n/
    │   ├── request.ts
    │   ├── routing.ts
    │   └── navigation.ts
    ├── proxy.ts          # Next.js 16+ (was middleware.ts)
    └── app/
        ├── layout.tsx    # Root layout, no NextIntlClientProvider here
        └── [locale]/
            ├── layout.tsx
            ├── page.tsx
            └── ...

Root layout does not wrap with NextIntlClientProvider; only app/[locale]/layout.tsx does.

1. Next config

Wire the plugin (default path ./i18n/request.ts):

// next.config.ts
import type { NextConfig } from "next";
import createNextIntlPlugin from "next-intl/plugin";

const nextConfig: NextConfig = {
  /* ... */
};
const withNextIntl = createNextIntlPlugin();
export default withNextIntl(nextConfig);

Custom path: createNextIntlPlugin('./src/i18n/request.ts').

2. Routing config

Central config in src/i18n/routing.ts:

import { defineRouting } from "next-intl/routing";

export const routing = defineRouting({
  locales: ["en", "ja", "zh-CN", "zh-TW"],
  defaultLocale: "en",
});

3. Request config

src/i18n/request.ts: resolve locale from the [locale] segment and load messages.

import { getRequestConfig } from "next-intl/server";
import { hasLocale } from "next-intl";
import { routing } from "./routing";

export default getRequestConfig(async ({ requestLocale }) => {
  const requested = await requestLocale;
  const locale = hasLocale(routing.locales, requested)
    ? requested
    : routing.defaultLocale;

  return {
    locale,
    messages: (await import(`../../messages/${locale}.json`)).default,
  };
});

4. Proxy / middleware (Next.js 16)

Next.js 16 uses proxy.ts instead of middleware.ts. Same API:

// src/proxy.ts
import createMiddleware from "next-intl/middleware";
import { routing } from "./i18n/routing";

export const proxy = createMiddleware(routing);

export const config = {
  matcher: "/((?!api|trpc|_next|_vercel|.*\\..*).*)",
};

Matcher: all pathnames except /api, /trpc, /next, /vercel, and paths containing a dot (e.g. favicon.ico).

5. Navigation helpers

Use project navigation wrappers so links keep the current locale:

// src/i18n/navigation.ts
import { createNavigation } from "next-intl/navigation";
import { routing } from "./routing";

export const { Link, redirect, usePathname, useRouter, getPathname } =
  createNavigation(routing);

In components: import Link (and others) from @/i18n/navigation, not from next/navigation or next/link, for locale-aware URLs.

6. Locale layout and static rendering

app/[locale]/layout.tsx must:

1. Validate locale with hasLocale → notFound() if invalid.
2. Call setRequestLocale(locale) for static rendering.
3. Wrap children with NextIntlClientProvider and getMessages().

// app/[locale]/layout.tsx
import { NextIntlClientProvider, hasLocale } from "next-intl";
import { setRequestLocale } from "next-intl/server";
import { notFound } from "next/navigation";
import { routing } from "@/i18n/routing";
import { getMessages } from "next-intl/server";

type Props = {
  children: React.ReactNode;
  params: Promise<{ locale: string }>;
};

export function generateStaticParams() {
  return routing.locales.map((locale) => ({ locale }));
}

export default async function LocaleLayout({ children, params }: Props) {
  const { locale } = await params;
  if (!hasLocale(routing.locales, locale)) notFound();

  setRequestLocale(locale);
  const messages = await getMessages();

  return (
    <NextIntlClientProvider messages={messages}>
      {children}
    </NextIntlClientProvider>
  );
}

7. Pages under [locale]

For static rendering, every page under [locale] that uses next-intl must call setRequestLocale(locale) (and use use(params) if needed). Layout already sets it; pages that render server components using locale should set it too.

// app/[locale]/page.tsx
import { use } from "react";
import { setRequestLocale } from "next-intl/server";

export default function IndexPage({
  params,
}: {
  params: Promise<{ locale: string }>;
}) {
  const { locale } = use(params);
  setRequestLocale(locale);
  return <TokyoPage />;
}

// app/[locale]/about/page.tsx
import { use } from "react";
import { setRequestLocale } from "next-intl/server";
import AboutContainer from "./components/AboutContainer";

export default function AboutPage({
  params,
}: {
  params: Promise<{ locale: string }>;
}) {
  const { locale } = use(params);
  setRequestLocale(locale);
  return <AboutContainer />;
}

Call setRequestLocale before any next-intl APIs in that layout/page.

8. Using translations

Client components: useTranslations(namespace):

"use client";
import { useTranslations } from "next-intl";
import { Link } from "@/i18n/navigation";

export default function BackToHomeButton() {
  const t = useTranslations("BackToHomeButton");
  return (
    <Link href="/">
      <span>{t("buttonText")}</span>
    </Link>
  );
}

"use client";
import { useTranslations } from "next-intl";
import { Link } from "@/i18n/navigation";

export default function Nav() {
  const t = useTranslations("Navigation");
  return <Link href="/about">{t("links.about")}</Link>;
}

Server components: use getTranslations from next-intl/server (await with locale/namespace as needed).

9. Messages format

One JSON file per locale under messages/. Nested keys map to namespaces and keys:

{
  "HomePage": {
    "title": "Hello world!"
  },
  "LandingPage": {
    "title": "Tokyo Sounds",
    "navbar": {
      "home": "Home",
      "about": "About"
    }
  },
  "BackToHomeButton": {
    "buttonText": "Back to Home",
    "tooltip": "Return to the main page"
  }
}

• useTranslations("LandingPage") → t("title"), t("navbar.about").
• Interpolation: "selectColor": "Select {color} color" → t("selectColor", { color: "Blue" }).

Checklist

• next.config.ts: createNextIntlPlugin() wraps config.
• src/i18n/routing.ts: defineRouting with locales and defaultLocale.
• src/i18n/request.ts: getRequestConfig + hasLocale + dynamic messages/${locale}.json.
• src/proxy.ts (or middleware.ts): createMiddleware(routing) and matcher.
• src/i18n/navigation.ts: createNavigation(routing) and re-export Link, etc.
• app/[locale]/layout.tsx: hasLocale → notFound, setRequestLocale, generateStaticParams, NextIntlClientProvider + getMessages().
• Each app/[locale]/**/page.tsx: setRequestLocale(locale) when using static rendering.
• Client components: useTranslations("Namespace"); links use Link from @/i18n/navigation.

Reference

• Extended config (localePrefix, pathnames, etc.): [reference.md](reference.md)
• Official: next-intl App Router, Routing setup