middleware.ts

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

acceptLanguage.languages(languages);

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

export function middleware(req: NextRequest) {
  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;

  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 refererHeaderValue = req.headers.get('referer');
    if (refererHeaderValue) {
      const refererUrl = new URL(refererHeaderValue);
      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();
}