middleware.ts

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

acceptLanguage.languages(languages);

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

const cookieName = "i18next";

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;
    }

    // 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: string) => refererUrl.pathname.startsWith(`/${l}`));
        const response = NextResponse.next();
        if (lngInReferer) {
            response.cookies.set(cookieName, lngInReferer);
        }
        return response;
    }

    return NextResponse.next();
}