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Internationalizing (i18n) with next-intl to make your Next.js 15 app multilingual

This guide walks you through next-intl best practices in a Next.js 15 (App Router) app, and shows how to layer Intlayer on top for robust translation management and automation.

See the comparison in next-i18next vs next-intl vs Intlayer.

For juniors: follow step-by-step sections to get a working multilingual app.
For mid-level devs: pay attention to payload optimization and server/client separation.
For seniors: note static generation, middleware, SEO integration, and automation hooks.

What we’ll cover:

Setup and file structure
Optimizing how messages are loaded
Client and server component usage
Metadata, sitemap, robots for SEO
Middleware for locale routing
Adding Intlayer on top (CLI and automation)

Set up your application using next-intl

Install the next-intl dependencies:

npm install next-intl

.├── locales│   ├── en│   │  ├── common.json│   │  └── about.json│   ├── fr│   │  ├── common.json│   │  └── about.json│   └── es│      ├── common.json│      └── about.json└── src    ├── i18n.ts    ├── middleware.ts    ├── app    │   └── [locale]    │       ├── layout.tsx    │       └── about    │           └── page.tsx    └── components        ├── ClientComponentExample.tsx        └── ServerComponent.tsx

Setup and Loading Content

Load only the namespaces your routes need and validate locales early. Keep server components synchronous when possible and push only the required messages to the client.

src/i18n.ts

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import { getRequestConfig } from "next-intl/server";import { notFound } from "next/navigation";export const locales = ["en", "fr", "es"] as const;export const defaultLocale = "en" as const;async function loadMessages(locale: string) {  // Load only the namespaces your layout/pages need  const [common, about] = await Promise.all([    import(`../locales/${locale}/common.json`).then((m) => m.default),    import(`../locales/${locale}/about.json`).then((m) => m.default),  ]);  return { common, about } as const;}export default getRequestConfig(async ({ locale }) => {  if (!locales.includes(locale as any)) notFound();  return {    messages: await loadMessages(locale),  };});

src/app/[locale]/layout.tsx

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import type { ReactNode } from "react";import { locales } from "@/i18n";import {  getLocaleDirection,  unstable_setRequestLocale,} from "next-intl/server";export const dynamic = "force-static";export function generateStaticParams() {  return locales.map((locale) => ({ locale }));}export default async function LocaleLayout({  children,  params,}: {  children: ReactNode;  params: { locale: string };}) {  const { locale } = params;  // Set the active request locale for this server render (RSC)  unstable_setRequestLocale(locale);  const dir = getLocaleDirection(locale);  return (    <html lang={locale} dir={dir}>      <body>{children}</body>    </html>  );}

src/app/[locale]/about/page.tsx

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import { getTranslations, getMessages, getFormatter } from "next-intl/server";import { NextIntlClientProvider } from "next-intl";import pick from "lodash/pick";import ServerComponent from "@/components/ServerComponent";import ClientComponentExample from "@/components/ClientComponentExample";export const dynamic = "force-static";export default async function AboutPage({  params,}: {  params: { locale: string };}) {  const { locale } = params;  // Messages are loaded server-side. Push only what's needed to the client.  const messages = await getMessages();  const clientMessages = pick(messages, ["common", "about"]);  // Strictly server-side translations/formatting  const tAbout = await getTranslations("about");  const tCounter = await getTranslations("about.counter");  const format = await getFormatter();  const initialFormattedCount = format.number(0);  return (    <NextIntlClientProvider locale={locale} messages={clientMessages}>      <main>        <h1>{tAbout("title")}</h1>        <ClientComponentExample />        <ServerComponent          formattedCount={initialFormattedCount}          label={tCounter("label")}          increment={tCounter("increment")}        />      </main>    </NextIntlClientProvider>  );}

Usage in a client component

Let's take an example of a client component rendering a counter.

Translations (shape reused; load them into next-intl messages as you prefer)

locales/en/about.json

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{  "counter": {    "label": "Counter",    "increment": "Increment"  }}

locales/fr/about.json

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{  "counter": {    "label": "Compteur",    "increment": "Incrémenter"  }}

Client component

src/components/ClientComponentExample.tsx

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"use client";import React, { useState } from "react";import { useTranslations, useFormatter } from "next-intl";const ClientComponentExample = () => {  // Scope directly to the nested object  const t = useTranslations("about.counter");  const format = useFormatter();  const [count, setCount] = useState(0);  return (    <div>      <p>{format.number(count)}</p>      <button        aria-label={t("label")}        onClick={() => setCount((count) => count + 1)}      >        {t("increment")}      </button>    </div>  );};

Don't forget to add "about" message on the page client message (only include the namespaces your client actually needs).

Usage in a server component

This UI component is a server component and can be rendered under a client component (page → client → server). Keep it synchronous by passing precomputed strings.

src/components/ServerComponent.tsx

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type ServerComponentProps = {  formattedCount: string;  label: string;  increment: string;};const ServerComponent = ({  formattedCount,  label,  increment,}: ServerComponentProps) => {  return (    <div>      <p>{formattedCount}</p>      <button aria-label={label}>{increment}</button>    </div>  );};

Notes:

Compute formattedCount server-side (e.g., const initialFormattedCount = format.number(0)).
Avoid passing functions or non-serializable objects into server components.

src/app/[locale]/about/layout.tsx

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import type { Metadata } from "next";import { locales, defaultLocale } from "@/i18n";import { getTranslations } from "next-intl/server";function localizedPath(locale: string, path: string) {  return locale === defaultLocale ? path : "/" + locale + path;}export async function generateMetadata({  params,}: {  params: { locale: string };}): Promise<Metadata> {  const { locale } = params;  const t = await getTranslations({ locale, namespace: "about" });  const url = "/about";  const languages = Object.fromEntries(    locales.map((locale) => [locale, localizedPath(locale, url)])  );  return {    title: t("title"),    description: t("description"),    alternates: {      canonical: localizedPath(locale, url),      languages: { ...languages, "x-default": url },    },  };}// ... Rest of the page code

src/app/sitemap.ts

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import type { MetadataRoute } from "next";import { locales, defaultLocale } from "@/i18n";const origin = "https://example.com";const formatterLocalizedPath = (locale: string, path: string) =>  locale === defaultLocale ? origin + path : origin + "/" + locale + path;export default function sitemap(): MetadataRoute.Sitemap {  const aboutLanguages = Object.fromEntries(    locales.map((l) => [l, formatterLocalizedPath(l, "/about")])  );  return [    {      url: formatterLocalizedPath(defaultLocale, "/about"),      lastModified: new Date(),      changeFrequency: "monthly",      priority: 0.7,      alternates: { languages: aboutLanguages },    },  ];}

src/app/robots.ts

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import type { MetadataRoute } from "next";import { locales, defaultLocale } from "@/i18n";const origin = "https://example.com";const withAllLocales = (path: string) => [  path,  ...locales    .filter((locale) => locale !== defaultLocale)    .map((locale) => "/" + locale + path),];export default function robots(): MetadataRoute.Robots {  const disallow = [    ...withAllLocales("/dashboard"),    ...withAllLocales("/admin"),  ];  return {    rules: { userAgent: "*", allow: ["/"], disallow },    host: origin,    sitemap: origin + "/sitemap.xml",  };}

Middleware for locale routing

Add a middleware to handle locale detection and routing:

src/middleware.ts

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import createMiddleware from "next-intl/middleware";import { locales, defaultLocale } from "@/i18n";export default createMiddleware({  locales: [...locales],  defaultLocale,  localeDetection: true,});export const config = {  // Skip API, Next internals and static assets  matcher: ["/((?!api|_next|.*\\..*).*)"],};

Best practices

Set html lang and dir: In src/app/[locale]/layout.tsx, compute dir via getLocaleDirection(locale) and set <html lang={locale} dir={dir}>.
Split messages by namespace: Organize JSON per locale and namespace (e.g., common.json, about.json).
Minimize client payload: On pages, send only required namespaces to NextIntlClientProvider (e.g., pick(messages, ['common', 'about'])).
Prefer static pages: Export export const dynamic = 'force-static' and generate static params for all locales.
Synchronous server components: Pass precomputed strings (translated labels, formatted numbers) rather than async calls or non-serializable functions.

Implement Intlayer on top of next-intl

Install the intlayer dependencies:

npm install intlayer @intlayer/sync-json-plugin  -D

Create the intlayer configuration file:

intlayer.config.ts

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import { type IntlayerConfig, Locales } from "intlayer";import { syncJSON } from "@intlayer/sync-json-plugin";const config: IntlayerConfig = {  internationalization: {    locales: [Locales.ENGLISH, Locales.FRENCH, Locales.SPANISH],    defaultLocale: Locales.ENGLISH,  },  ai: {    apiKey: process.env.OPENAI_API_KEY,  },  plugins: [    // Keep your per-namespace folder structure in sync with Intlayer    syncJSON({      source: ({ key, locale }) => `./locales/${locale}/${key}.json`,    }),  ],};export default config;

Add package.json scripts:

package.json

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{  "scripts": {    "i18n:fill": "intlayer fill",    "i18n:test": "intlayer test"  }}

Notes:

intlayer fill: uses your AI provider to fill missing translations based on your configured locales.
intlayer test: checks for missing/invalid translations (use it in CI).

You can configure arguments and providers; see Intlayer CLI.

Warum Intlayer?