Simply way to support multiple languages i18n in Next.js 14 (Based on App Router)

8 min readApr 28, 2024

Introduction:

This post introduces the implementation of i18n internationalization multi-language feature in Next.js 14 (based on App Router), and takes into consideration actual scenarios to optimize and perfect the feature step-by-step. By reading this post, you will immediately get how to implement i18n in Next.js.

Preface

In an era where the Internet world is becoming increasingly flattening, the importance of multi-language products is growing. Fortunately, Next.js allows us to quickly support multi-language with simple configurations and code. However, when we search for how Next.js supports multi-languages on the Internet, we might find a variety of implementations, jumbled information, and excessively clever solutions. Then we become confused and start to question: What’s the issue here?

Today, let’s implement multi-language from 0 to 1 in Next.js and unravel the mystery of multi-language.

We can refer to the i18n introduction in the Next.js official documentation here: https://nextjs.org/docs/app/building-your-application/routing/internationalization, which is quite clear and detailed. This article will be based on this documentation.

Before we begin, let’s take a look at the final running effect: https://next-i18n-demo-two.vercel.app/

Ready to work

First, We create a Next.js app,

npx create-next-app@latest

Plese select App Router, I am using TypeScript here.

❯ npx create-next-app@latest
✔ What is your project named? … `next-i18n-demo`
✔ Would you like to use TypeScript? … No / `Yes`
✔ Would you like to use ESLint? … No / `Yes`
✔ Would you like to use Tailwind CSS? … No / `Yes
✔ Would you like to use `src/` directory? … No / `Yes`
✔ Would you like to use App Router? (recommended) … No / `Yes`
✔ Would you like to customize the default import alias (@/*)? … `No` / Yes

Run locally,

npm run dev

Open http://localhost:3000 and you will see that it is running okay.

Internationalization introduction

Before we start it, let’s briefly introduce about internationalization. internationalization, aka i18n, this means supporting multiple languages, cultures, and customs in products, mainly including language, time, currency symbols, etc. This article will focus only on the language part.

In terms of internationalization, a common approach is for a website to default to a certain language’s official site (usually English), and to support the selection of language or region, allowing for a switch to different language versions of the site.

Specifically, some websites use a language abbreviation as a prefix, such as en.wikipedia.org, zh.wikipedia.org; some use it as a path suffix, such as aws.amazon.com/cn, aws.amazon.com/jp, and others distinguish based on the country or region domain name, such as apple.cn, apple.jp.

Among these, en, zh, cn, jp, etc., are language codes, which can vary slightly in different versions. You can refer to the reference materials at the end of the article for specifics.

In this article’s particular case, the ISO_3166 codes en and zh will be used to represent English and Chinese respectively.

Begin to configure Multi-Languages

The original file structure of the project was:

├── package.json
├── public
│   ├── next.svg
│   └── vercel.svg
├── src
│   └── app
│       ├── favicon.ico
│       ├── globals.css
│       ├── layout.tsx
│       └── page.tsx
├── tailwind.config.ts
└── tsconfig.json

We create a new folder named [lang] in the app directory, then move laytout.tsx and page.tsx from the app directory to [locales].

The file structure after moving is:

├── package.json
├── postcss.config.mjs
├── public
│   ├── next.svg
│   └── vercel.svg
├── src
│   └── app
│       ├── [lang]
│       │   ├── layout.tsx
│       │   └── page.tsx
│       ├── favicon.ico
│       └── globals.css
├── tailwind.config.ts
└── tsconfig.json

Tips:
Please modify the reference position of globals.css in layout.tsx.

Next, we define json resource files for different languages, which you can put in your preferred file directory. I put it in public/dictionaries. The file format is as follows:

en.json

{
  "page": {
    "title": "Next.js i18n Demo",
    "desc": "How to implement i18n with Next.js (based on App Router)"
  },
  "home": {
    "title": "Hello, Next.js i18n",
    "desc": "This is a demo of Next.js i18n"
  }
}

zh.json

{
  "page": {
    "title": "Next.js i18n 示例",
    "desc": "搞懂 Next.js 实现 i18n 国际化多语言(基于App Router)"
  },
  "home": {
    "title": "你好, Next.js i18n",
    "desc": "这是一个 Next.js i18n 示例"
  }
}

Then, we create a file to load the multi-language resource files and get the corresponding language text.

Add dictionaries.js in the app/[lang] directory. Make sure the file directory and file name are correct and match.

import 'server-only'
 
const dictionaries = {
  en: () => import('./dictionaries/en.json').then((module) => module.default),
  zh: () => import('./dictionaries/zh.json').then((module) => module.default),
}
 
export const getDictionary = async (locale) => dictionaries[locale]()

Using Multi-Languages

We use the multi-language feature on the pages.tsx page.

First, add the lang parameter for the function, add async for the function,

export default async function Home({ params: { lang } }: { params: { lang: string } }) {
  ...
}

Use it on the page, add the multi-language fuction call,

const t = await getDictionary(lang);

For convenience, I clean up the default code on page.tsx and only retain the text display.

<main className="flex min-h-screen flex-col items-center justify-between p-24">
      <p className="fixed left-0 top-0 flex w-full justify-center border-b border-gray-300 bg-gradient-to-b from-zinc-200 pb-6 pt-8 backdrop-blur-2xl dark:border-neutral-800 dark:bg-zinc-800/30 dark:from-inherit lg:static lg:w-auto  lg:rounded-xl lg:border lg:bg-gray-200 lg:p-4 lg:dark:bg-zinc-800/30">
          {t.home.title}
      </p>
      {t.home.desc}
</main>

Restart the program or wait for the program to hot update successfully, open different language pages http://localhost:3000/en http://localhost:3000/zh to check the effect.

Setting the Default Language

It looks pretty good, but careful friends will find that opening http://localhost:3000 will result in a 404 error. To solve this problem, we need to set a default language when no language is selected.

For this, we can create a middleware.ts in the src directory, and then copy the code from the documentation.

The core logic is simple:

Check whether there is a certain language identifier in URL’s pathname. If so, return directly. Otherwise, get the appropriate language and redirect the URL to /${locale}${pathname}

The focus is on the getLocale function. We need to specify the suitable language. For now, let's deal with this simply: use the default defaultLocale = "en" .

import { NextRequest, NextResponse } from "next/server";

let locales = ["en", "zh"];
let defaultLocale = "en";

// Get the preferred locale, similar to the above or using a library
function getLocale(request: NextRequest) {
  return defaultLocale;
}

export function middleware(request: NextRequest) {
  // Check if there is any supported locale in the pathname
  const { pathname } = request.nextUrl;
  const pathnameHasLocale = locales.some(
    (locale) => pathname.startsWith(`/${locale}/`) || pathname === `/${locale}`
  );

  if (pathnameHasLocale) return;

  // Redirect if there is no locale
  const locale = getLocale(request);
  request.nextUrl.pathname = `/${locale}${pathname}`;
  // e.g. incoming request is /products
  // The new URL is now /en-US/products
  return NextResponse.redirect(request.nextUrl);
}

export const config = {
  matcher: [
    // Skip all internal paths (_next)
    "/((?!_next).*)",
    // Optional: only run on root (/) URL
    // '/'
  ],
};

After the program updates, we open http://localhost:3000/ and see that it will automatically redirect to the default language page.

Optimization of Getting the Default Language

In the previous step, while get the default language, we treated it simply as defaultLocale = "en". A more graceful way is: Set the default language based on the user's system or browser language.

We can achieve this by getting the Accept-Language field from the browser's HTTP headers. The data format is approximately as follows:

English:
accept-language: en-US,en;q=0.5
Chinese:
accept-language: zh-CN,zh-Hans;q=0.9

We update middleware as follows:

Get the Accept-Language from the HTTP headers. If it's empty, then return the default language.
Parse the language list in Accept-Language and match to get the corresponding language based on the configured language list. (If there is no match, return the default language)

Install dependencies @formatjs/intl-localematcher, negotiator, @types/negotiator, and implement the following logic:

function getLocale(request: NextRequest) {
  const acceptLang = request.headers.get("Accept-Language");
  if (!acceptLang) return defaultLocale;
  const headers = { "accept-language": acceptLang };
  const languages = new Negotiator({ headers }).languages();
  return match(languages, locales, defaultLocale);
}

By changing the system language, open http://localhost:3000 and it will automatically redirect to the page with the same system language. Test successfully.

Other Handling of Multi-Language

Storing the Language

Going a step further, we can store the user’s web page language in the cookies and use it on the next visit:

// Get Cookie 
if (request.cookies.has(cookieName)) {
  return request.cookies.get(cookieName)!.value;
}

// Set Cookie
response.cookies.set(cookieName, locale);

Web Metadata(Page Title/Descriptions..)

When using i18n in web page metadata, add the following code to page.tsx:

export async function generateMetadata({ params: { lang } } : { params: { lang: string } }) {
  const t = await getDictionary(lang);
  return {
    title: t.page.title,
    description: t.page.desc,
  };
}

SSG(Static Generation)

When handling i18n in SSG, the code in layout.tsx is as follows:

interface LangParams {
  lang: string;
}

export async function generateStaticParams() {
  return [{ lang: "en" }, { lang: "zh" }];
}

export default function RootLayout({
  children,
  params,
}: Readonly<{
  children: React.ReactNode;
  params: LangParams;
}>) {
  return (
    <html lang={params.lang}>
      <body className={inter.className}>{children}</body>
    </html>
  );
}

Language Switch（Language Switcher or Links）

You can add a language swicher (like a drop-down menu) or some links.

For example,

<div className="space-x-2">
  <Link href="/en">English</Link>
  <span>|</span>
  <Link href="/zh">Chinese</Link>
</div>

End

Through the learning of the above steps, we initially familiarize and practice using multi-language in Next.js. A journey of thousand miles begins with a single step. The work of i18n is not limited to these, and of course, other areas need improvement which I’d leave to you, the reader.

Finally, here is the complete code of middleware.ts :

import Negotiator from "negotiator";
import { match } from "@formatjs/intl-localematcher";
import { NextRequest, NextResponse } from "next/server";

const locales = ["en", "zh"];
const defaultLocale = "zh";
const cookieName = "i18nlang";

// Get the preferred locale, similar to the above or using a library
function getLocale(request: NextRequest): string {
  // Get locale from cookie
  if (request.cookies.has(cookieName))
    return request.cookies.get(cookieName)!.value;
  // Get accept language from HTTP headers
  const acceptLang = request.headers.get("Accept-Language");
  if (!acceptLang) return defaultLocale;
  // Get match locale
  const headers = { "accept-language": acceptLang };
  const languages = new Negotiator({ headers }).languages();
  return match(languages, locales, defaultLocale);
}

export function middleware(request: NextRequest) {
  if (request.nextUrl.pathname.startsWith("/_next")) return NextResponse.next();

  // Check if there is any supported locale in the pathname
  const { pathname } = request.nextUrl;
  const pathnameHasLocale = locales.some(
    (locale) => pathname.startsWith(`/${locale}/`) || pathname === `/${locale}`
  );

  if (pathnameHasLocale) return;

  // Redirect if there is no locale
  const locale = getLocale(request);
  request.nextUrl.pathname = `/${locale}${pathname}`;
  // e.g. incoming request is /products
  // The new URL is now /en-US/products
  const response = NextResponse.redirect(request.nextUrl);
  // Set locale to cookie
  response.cookies.set(cookieName, locale);
  return response;
}

export const config = {
  matcher: [
    // Skip all internal paths (_next)
    "/((?!_next).*)",
    // Optional: only run on root (/) URL
    // '/'
  ],
};