middleware.js

import { NextResponse } from 'next/server'
import acceptLanguage from 'accept-language'
import { fallbackLng, languages, cookieName } from './app/i18n/settings'

acceptLanguage.languages(languages)

export const config = {
  matcher: [
    /**
     * Match all request paths except for the ones starting with:
     * - api (API routes)
     * - _next/static (static files)
     * - _next/image (image optimization files)
     * - favicon.ico (favicon file)
     * - sw.js (service worker)
     * - next.svg (Next.js logo)
     * - vercel.svg (Vercel logo)
     */
    '/((?!api|_next/static|_next/image|assets|img|images|favicon.ico|sw.js|next.svg|vercel.svg).*)',
  ],
}

export function middleware(req) {
  let lng
  if (req.cookies.has(cookieName)) lng = acceptLanguage.get(req.cookies.get(cookieName).value)
  if (!lng) lng = acceptLanguage.get(req.headers.get('Accept-Language'))
  if (!lng) lng = fallbackLng

  // Redirect if lng in path is not supported
  if (
    !languages.some(loc => req.nextUrl.pathname.startsWith(`/${loc}`)) &&
    !req.nextUrl.pathname.startsWith('/_next')
  ) {
    return NextResponse.redirect(new URL(`/${lng}${req.nextUrl.pathname}`, req.url))
  }

  if (req.headers.has('referer')) {
    const refererUrl = new URL(req.headers.get('referer'))
    const lngInReferer = languages.find((l) => refererUrl.pathname.startsWith(`/${l}`))
    const response = NextResponse.next()
    if (lngInReferer) response.cookies.set(cookieName, lngInReferer)
    return response
  }

  return NextResponse.next()
}