NextJS i18n/Internationalization

Table of Contents

• TL;DR Source and Demo
• Introduction
• Project Setup
• Internationalized Routing
• Content translation
• Built-in Formatting
• Fetching from backend

TL;DR

Check the demo here
Check the source code here

Introduction

Internationalization (i18n) is the process of preparing software so that it can support local languages and cultural settings. An internationalized product supports the requirements of local markets around the world, functioning more appropriately based on local norms and better meeting in-country user expectations. Copy-pasted from here

In my early days of development, I find i18n to be a tedious task. However, in NextJS, it is relatively simple to create such as challenging feature.

Project Setup

Initialize a NextJS project

Let's start by creating a new NextJS project. The simplest way is to use these commands:

npx create-next-app@latest
# or
yarn create next-app

For more information, check this Create Next App docs

Remove boilerplate code

Let's simplify the project by removing unused code.

// pages/index.jsx
export default function Home() {
  return <main>Hello world</main>;
}

Check the changes here

Create another route/page

This step is not related to i18n. It is mainly for easy demonstration.

Update the Home page to display the current locale.

// pages/index.jsx
import { useRouter } from "next/router";

export default function Home() {
  const { locale } = useRouter();

  return <main>Hello world: {locale}</main>;
}

Let's create an About page with the same content as the Home page.

// pages/about.jsx
import { useRouter } from "next/router";

export default function About() {
  const { locale } = useRouter();

  return <main>About page: {locale}</main>;
}

Without any configuration changes, the pages will be rendered as:

As you can see, localhost:3000 shows Hello world:. This is because useRouter is not aware of the value of locale.

localhost:3000/zh-CN and localhost:3000/sv obviously will not exist because we have not created pages/zh-CN.jsx and pages/sv.jsx

Internationalized Routing

Built-in NextJS i18n routing

Let's add this simple i18n configuration to our next.config.js file and see what happens.

// next.config.js
const nextConfig = {
  // other stuff
  i18n: {
    defaultLocale: "en",
    locales: ["en", "sv", "zh-CN"],
  },
};

With the configuration above, we automagically get the locale value and the following routes:

Home page

About page

Not defined locale

If you try to access localhost:3000/fr, you will still get a 404 error. This is because we did not add fr to our locale values

Create a header component

To further simplify our demo, let's create a header component that can:

• Navigate to home and about pages
• Change the locale values using a dropdown

// components/Header.jsx
import React from "react";
import Link from "next/link";
import { useRouter } from "next/router";

const Header = () => {
  const router = useRouter();

  const handleLocaleChange = (event) => {
    const value = event.target.value;

    router.push(router.route, router.asPath, {
      locale: value,
    });
  };

  return (
    <header>
      <nav>
        <Link href="/">
          <a className={router.asPath === "/" ? "active" : ""}>Home</a>
        </Link>
        <Link href="/about">
          <a className={router.asPath === "/about" ? "active" : ""}>About</a>
        </Link>
      </nav>

      <select onChange={handleLocaleChange} value={router.locale}>
        <option value="en">🇺🇸 English</option>
        <option value="zh-CN">🇨🇳 中文</option>
        <option value="sv">🇸🇪 Swedish</option>
      </select>

      <style jsx>{`
        a {
          margin-right: 0.5rem;
        }

        a.active {
          color: blue;
        }

        nav {
          margin-bottom: 0.5rem;
        }
      `}</style>
    </header>
  );
};

export default Header;

Let's add the Header component to our pages/_app.js file.

// pages/_app.jsx
import Header from "../components/Header";
import "../styles/globals.css";

function MyApp({ Component, pageProps }) {
  return (
    <>
      <Header />
      <Component {...pageProps} />
    </>
  );
}

export default MyApp;

Now we can see clearly the power of NextJS built-in i18n support. We can now access the locale value in our useRouter hook, and the URL is updated based on the locale.

To learn more about NextJS i18n routing, check this link.

Content translation

Unfortunately, there is no NextJS built-in support for content translation so we need to do it on our own.

However, there are a libraries that can help to not reinvent the wheel. In this blog post, we will use next-i18next.

Let's support content translation by setting up next-i18next in our app.

Install next-i18next

npm install next-i18next

Create a next-i18next.config.js and update next.config.js

// next-i18next.config.js
module.exports = {
  i18n: {
    defaultLocale: "en",
    locales: ["en", "sv", "zh-CN"],
    localePath: "./locales",
  },
};

localePath is optional and will default to ./public/locales.

// next.config.js
const { i18n } = require("./next-i18next.config");

const nextConfig = {
  // other stuff
  i18n,
};

module.exports = nextConfig;

Create translation files

.
└── locales
    ├── en
    |   └── common.json
    |   └── home.json
    └── zh-CH
    |   └── common.json
    |   └── home.json
    └── se
        └── common.json
        └── home.json

English Translations

// locales/en/common.json
{
  "greeting": "Hello world!"
}

// locales/en/home.json
{
  "home": "Home",
  "about": "About"
}

Please forgive me for any translation mistakes. I only used Google Translate to translate the content. 🤣

Chinese translations

// locales/zh-CN/common.json
{
  "greeting": "世界您好"
}

// locales/zh-CN/home.json
{
  "home": "主页",
  "about": "关于页面"
}

Swedish translations

// locales/sv/common.json
{
  "greeting": "Hej världen!"
}

// locales/sv/home.json
{
  "home": "Hem",
  "about": "Om"
}

There are three functions that next-i18next exports, which you will need to use to translate your project:

appWithTranslation

This is a HOC which wraps your _app. This HOC is primarily responsible for adding a I18nextProvider.

// pages/_app.jsx
import { appWithTranslation } from "next-i18next";
import Header from "../components/Header";
import "../styles/globals.css";

function MyApp({ Component, pageProps }) {
  return (
    <>
      <Header />
      <Component {...pageProps} />
    </>
  );
}

export default appWithTranslation(MyApp);

serverSideTranslations

This is an async function that you need to include on your page-level components, via either getStaticProps or getServerSideProps.

// pages/index.jsx
import { serverSideTranslations } from "next-i18next/serverSideTranslations";

// export default function Home...

export async function getStaticProps({ locale }) {
  return {
    props: {
      ...(await serverSideTranslations(locale, ["common", "home"])),
      // Will be passed to the page component as props
    },
  };
}

useTranslation

This is the hook which you'll actually use to do the translation itself. The useTranslation hook comes from react-i18next, but can be imported from next-i18next directly:

// pages/index.jsx
// other imports
import { useTranslation } from "next-i18next";

export default function Home() {
  // We want to get the translations from `home.json`
  const { t } = useTranslation("home");

  // Get the translation for `greeting` key
  return <main>{t("greeting")}</main>;
}

// export async function getStaticProps...

Let's also translate the links in the Header component.

// components/Header.jsx
// other imports
import { useTranslation } from "next-i18next";

const Header = () => {
  // ...

  // If no argument is passed, it will use `common.json`
  const { t } = useTranslation();

  return (
    <header>
      <nav>
        <Link href="/">
          <a className={router.asPath === "/" ? "active" : ""}>{t("home")}</a>
        </Link>
        <Link href="/about">
          <a className={router.asPath === "/about" ? "active" : ""}>
            {t("about")}
          </a>
        </Link>
      </nav>
      {/* Other code */}
    </header>
  );
}

The changes above will yield the following output:

The home page is translated properly; however, the about page is not. It is because we need to use serverSideTranslations in every route.

// pages/about.jsx
// other imports
import { serverSideTranslations } from "next-i18next/serverSideTranslations";

// export default function About...

export async function getStaticProps({ locale }) {
  return {
    props: {
      ...(await serverSideTranslations(locale, ["common"])),
    },
  };
}

Now both routes are translated

We only specified common in the serverSideTranslations because we don't plan on using anything in home.json in the About page.

I will fetch the translations of the About page's content from the backend. But before that, let's first check some cool stuff we can do with our translation library.

Nested translation keys and default translation

We are not limited to a flat JSON structure.

// locales/en/newsletter.json
{
  "title": "Stay up to date",
  "subtitle": "Subscribe to my newsletter",
  "form": {
    "firstName": "First name",
    "email": "E-mail",
    "action": {
      "signUp": "Sign Up",
      "cancel": "Cancel"
    }
  }
}

We can omit some translation keys if we want it to use the default locale value(en in our case).

// locales/zh-CN/newsletter.json
{
  "title": "保持最新状态",
  "form": {
    "email": "电子邮箱",
    "action": {
      "cancel": "取消"
    }
  }
}

Let's create a component which use the translations above.

// components/SubscribeForm.jsx
import { useTranslation } from "next-i18next";
import React from "react";

const SubscribeForm = () => {
  const { t } = useTranslation("newsletter");

  return (
    <section>
      <h3>{t("title")}</h3>
      <h4>{t("subtitle")}</h4>

      <form>
        <input placeholder={t("form.firstName")} />
        <input placeholder={t("form.email")} />
        <button>{t("form.action.signUp")}</button>
        <button>{t("form.action.cancel")}</button>
      </form>

      {/* For styling only */}
      <style jsx>{`
        form {
          max-width: 300px;
          display: flex;
          flex-direction: column;
        }

        input {
          margin-bottom: 0.5rem;
        }
      `}</style>
    </section>
  );
};

export default SubscribeForm;

Render the form in pages/index.jsx and add newsletter in serverSideTranslations.

// pages/index.jsx
import { serverSideTranslations } from "next-i18next/serverSideTranslations";
import { useTranslation } from "next-i18next";
import SubscribeForm from "../components/SubscribeForm";

export default function Home() {
  const { t } = useTranslation("home");

  return (
    <main>
      <div>{t("greeting")}</div>
      {/* Render the form here */}
      <SubscribeForm />
    </main>
  );
}

export async function getStaticProps({ locale }) {
  return {
    props: {
      ...(await serverSideTranslations(locale, [
        "common",
        "home",
        "newsletter", // Add newsletter translations
      ])),
    },
  };
}

And now, we have this!

Built-in Formatting

It is very easy to format most of our data since next-i18next is using i18next under the hood.

• Number
• Currency
• DateTime
• RelativeTime
• List

Let's use the translation files below to showcase the formatting features.

// locales/en/built-in-demo.json
{
  "number": "Number: {{val, number}}",
  "currency": "Currency: {{val, currency}}",
  "dateTime": "Date/Time: {{val, datetime}}",
  "relativeTime": "Relative Time: {{val, relativetime}}",
  "list": "List: {{val, list}}",
  "weekdays": ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]
}

// locales/zh-CN/built-in-demo.json
{
  "number": "数: {{val, number}}",
  "currency": "货币: {{val, currency}}",
  "dateTime": "日期/时间: {{val, datetime}}",
  "relativeTime": "相对时间: {{val, relativetime}}",
  "list": "列表: {{val, list}}",
  "weekdays": ["星期一", "星期二", "星期三", "星期四", "星期五"]
}

// locales/sv/built-in-demo.json
{
  "number": "Nummer: {{val, number}}",
  "currency": "Valuta: {{val, currency}}",
  "dateTime": "Datum/tid: {{val, datetime}}",
  "relativeTime": "Relativ tid: {{val, relativetime}}",
  "list": "Lista: {{val, list}}",
  "weekdays": ["Måndag", "Tisdag", "Onsdag", "Torsdag", "Fredag"]
}

Let's create a component which use the translations above.

import { useTranslation } from "next-i18next";
import React from "react";

const BuiltInFormatsDemo = () => {
  const { t } = useTranslation("built-in-demo");

  return (
    <div>
      <p>
        {/* "number": "Number: {{val, number}}", */}
        {t("number", {
          val: 123456789.0123,
        })}
      </p>
      <p>
        {/* "currency": "Currency: {{val, currency}}", */}
        {t("currency", {
          val: 123456789.0123,
          style: "currency",
          currency: "USD",
        })}
      </p>

      <p>
        {/* "dateTime": "Date/Time: {{val, datetime}}", */}
        {t("dateTime", {
          val: new Date(1234567890123),
          formatParams: {
            val: {
              weekday: "long",
              year: "numeric",
              month: "long",
              day: "numeric",
            },
          },
        })}
      </p>

      <p>
        {/* "relativeTime": "Relative Time: {{val, relativetime}}", */}
        {t("relativeTime", {
          val: 12,
          style: "long",
        })}
      </p>

      <p>
        {/* "list": "List: {{val, list}}", */}
        {t("list", {
          // https://www.i18next.com/translation-function/objects-and-arrays#objects
          // Check the link for more details on `returnObjects`
          val: t("weekdays", { returnObjects: true }),
        })}
      </p>
    </div>
  );
};

export default BuiltInFormatsDemo;

The more you look, the more you'll be amazed

Other translation functions to check

• More info about Interpolation
• Nesting
• Pluralization
• Context

Fetching translations from backend

The work here is mainly done on the backend side or your CMS. On the frontend, we simply fetch the translations and pass a parameter to distinguish the language we want.

I created a simple endpoint to fetch the content of the about page. The result will change based on query param lang value.

// pages/api/about.js
export default function handler(req, res) {
  const lang = req.query.lang || "en";

  if (lang === "sv") {
    return res.status(200).json({ message: "Jag är Code Gino" });
  } else if (lang === "zh-CN") {
    return res.status(200).json({ message: "我是代码吉诺" });
  } else {
    return res.status(200).json({ message: "I am Code Gino" });
  }
}

Sample usage

• /api/about: English
• /api/about?lang=zh-CN: Simplified Chinese
• /api/about?lang=sv: Svenska
• /api/about?lang=invalid: English

We can consume the API as usual (e.g. inside getServerSideProps, getStaticProps, useEffect, etc.).

In this example, let's fetch the translation inside the getStaticProps. We can get the locale value from the context, then append ?lang=${locale} to our request URL.

// pages/about.jsx
// This import is not related to fetching translations from backend.
import { serverSideTranslations } from "next-i18next/serverSideTranslations";

export default function About({ message }) {
  return <h1>{message}</h1>;
}

export async function getStaticProps({ locale }) {
  const { message } = await fetch(
    // forward the locale value to the server via query params
    `https://next-i18n-example-cg.vercel.app/api/about?lang=${locale}`
  ).then((res) => res.json());

  return {
    props: {
      message,
      // The code below is not related to fetching translations from backend.
      ...(await serverSideTranslations(locale, ["common"])),
    },
  };
}

The code above will yield the following result:

Conclusion

Internationalization is a complex requirement simplified in Next.js due to the built-in i18n routing support and the easy integration of next-i18next. And because next-i18next is using i18next, we can perform better translations with less code.