middleware.ts

import { NextRequest, NextResponse } from "next/server";
import acceptLanguage from 'accept-language'
import { fallbackLng, languages, cookieName } from '@/i18n/settings'

acceptLanguage.languages(languages)

export const config = {
  // matcher: '/:lng*'
  matcher: ['/((?!api|_next/static|_next/image|assets|favicon.ico|sw.js).*)']
}

const PUBLIC_FILE = /\.(.*)$/;

export async function middleware(req: NextRequest) {
  if (
    req.nextUrl.pathname.startsWith("/_next") ||
    req.nextUrl.pathname.includes("/api/") ||
    PUBLIC_FILE.test(req.nextUrl.pathname)
  ) {
    return;
  }
  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()
}