Persisting a user's language choice

Problem

A user manually selects 'French' on a site. When they close their browser and later type in the site's main address (e.g., example.com), the application reverts to its default (e.g., English) or auto-detected language. This failure to remember their choice forces the user to find the language switcher and re-select their language every time they start a new session.

Solution

When a user selects a language, store that choice in a cookie. In the middleware, when a user visits the root path (/), check for this cookie before checking the Accept-Language header. If a valid cookie is found, redirect the user to their chosen language's root (e.g., /fr), overriding any browser default.

Steps

1. Define language configuration

Create a central configuration file to store your supported languages, a default language, and the name of the cookie you will use to store the user's preference.

// i18n.config.ts

export const locales = ['en', 'es', 'fr'];
export const defaultLocale = 'en';
export const localeCookieName = 'NEXT_LOCALE';

2. Install a language parser

You still need a parser for the Accept-Language header, which will be used as a fallback if no cookie is set.

npm install accept-language-parser

3. Create the middleware

Create a middleware.ts file at the root of your project. This middleware will check for the NEXT_LOCALE cookie first. If it's not found, it will fall back to checking the Accept-Language header. This logic will only apply to requests for the root path (/).

// middleware.ts
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';
import parser from 'accept-language-parser';
import {
  locales,
  defaultLocale,
  localeCookieName,
} from './i18n.config';

function getPreferredLocale(request: NextRequest) {
  // 1. Check for the cookie
  const cookie = request.cookies.get(localeCookieName);
  if (cookie) {
    const locale = cookie.value;
    if (locales.includes(locale)) {
      return locale;
    }
  }

  // 2. Check the Accept-Language header
  const acceptLang = request.headers.get('Accept-Language');
  if (acceptLang) {
    const bestMatch = parser.pick(locales, acceptLang, {
      loose: true,
    });
    if (bestMatch) {
      return bestMatch;
    }
  }

  // 3. Return the default
  return defaultLocale;
}

export function middleware(request: NextRequest) {
  const { pathname } = request.nextUrl;

  // 1. Check if the request is for the root path
  if (pathname === '/') {
    // Get the user's preferred language (from cookie or header)
    const bestLocale = getPreferredLocale(request);

    // Redirect to the best-matched language path
    request.nextUrl.pathname = `/${bestLocale}`;
    return NextResponse.redirect(request.nextUrl);
  }

  // 2. For all other paths, continue as normal
  return NextResponse.next();
}

export const config = {
  matcher: [
    // We only want to run this on the root path for now
    '/',
    // We also need to match non-root paths to let them pass
    '/((?!api|_next/static|_next/image|favicon.ico).*)',
  ],
};

This middleware's logic now correctly prioritizes the user's explicit choice (the cookie) over their implicit preference (the browser header) when they visit the site's root.