构建双语 Next.js App Router 网站：把 i18n、hreflang 和结构化数据做对

app/
  [locale]/
    layout.tsx       ← root layout: <html lang>, nav, footer, JSON-LD, hreflang
    page.tsx         ← home
    services/page.tsx
    work/
      page.tsx       ← work index
      [slug]/page.tsx← case study
    studio/page.tsx
    contact/page.tsx
    insights/
      page.tsx
      [slug]/page.tsx
    [...rest]/page.tsx ← catch-all → notFound()
    not-found.tsx    ← localized 404
  sitemap.ts
  robots.ts

// middleware.ts
import { NextResponse, type NextRequest } from "next/server";
import { defaultLocale, isLocale, locales } from "@/lib/i18n";

const LOCALE_COOKIE = "neptay.locale";

function pickLocaleFromAcceptLanguage(header: string | null): "en" | "tr" {
  if (!header) return defaultLocale;
  const lower = header.toLowerCase();
  if (/(^|[,;\s])tr(-|;|,|$)/.test(lower)) return "tr";
  return defaultLocale;
}

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

  // Live demos live outside the locale system — they handle their own language.
  if (pathname === "/demo" || pathname.startsWith("/demo/")) {
    return NextResponse.next();
  }

  const matchedLocale = locales.find(
    (l) => pathname === `/${l}` || pathname.startsWith(`/${l}/`)
  );
  if (matchedLocale) {
    const res = NextResponse.next();
    // Remember the locale so the next bare-domain visit lands on it.
    if (req.cookies.get(LOCALE_COOKIE)?.value !== matchedLocale) {
      res.cookies.set(LOCALE_COOKIE, matchedLocale, {
        path: "/",
        maxAge: 60 * 60 * 24 * 365,
        sameSite: "lax",
      });
    }
    return res;
  }

  // Honour the user's manual choice if it was already remembered.
  const cookieLocale = req.cookies.get(LOCALE_COOKIE)?.value;
  const remembered = cookieLocale && isLocale(cookieLocale) ? cookieLocale : null;
  const detected = remembered ?? pickLocaleFromAcceptLanguage(req.headers.get("accept-language"));

  const url = req.nextUrl.clone();
  url.pathname = `/${detected}${pathname === "/" ? "" : pathname}`;
  return NextResponse.redirect(url);
}

export const config = {
  // Skip Next internals + static assets so they aren't redirected.
  matcher: [
    "/((?!_next/|api/|favicon\\.|robots\\.txt|sitemap\\.xml|.*\\.(?:png|svg|jpg|jpeg|gif|webp|ico|txt|xml)).*)",
  ],
};

export async function generateMetadata({ params }: { params: { locale: string } }): Promise<Metadata> {
  if (!isLocale(params.locale)) return {};
  const seo = WORK_SEO[params.locale];
  return {
    title: seo.title,
    description: seo.description,
    alternates: {
      canonical: `/${params.locale}/work`,
      languages: {
        en: "/en/work",
        tr: "/tr/work",
        "x-default": "/en/work",
      },
    },
    openGraph: { title: seo.title, description: seo.description, type: "website" },
  };
}

// app/sitemap.ts
import type { MetadataRoute } from "next";
import { locales } from "@/lib/i18n";

const SITE = "https://neptay.com";
const PATHS = ["", "services", "work", "studio", "contact", "insights"] as const;

export default function sitemap(): MetadataRoute.Sitemap {
  const lastModified = new Date();
  return locales.flatMap((locale) =>
    PATHS.map((p) => ({
      url: `${SITE}/${locale}${p ? `/${p}` : ""}`,
      lastModified,
      changeFrequency: "monthly" as const,
      priority: p === "" ? 1 : 0.7,
      alternates: {
        languages: Object.fromEntries(
          locales.map((l) => [l, `${SITE}/${l}${p ? `/${p}` : ""}`])
        ),
      },
    }))
  );
}

// lib/structured-data.ts
export const organizationLd = {
  "@type": "Organization",
  "@id": `${SITE_URL}/#organization`,
  name: "Neptay",
  url: SITE_URL,
  logo: { "@type": "ImageObject", url: `${SITE_URL}/neptay-logo.png` },
  knowsLanguage: ["en", "tr"],
  areaServed: "Worldwide",
};

export function articleLd(input: ArticleInput) {
  return {
    "@context": "https://schema.org",
    "@type": "Article",
    headline: input.headline,
    inLanguage: input.locale,            // ← critical for bilingual sites
    datePublished: input.datePublished,
    author: { "@id": `${SITE_URL}/#organization` },
    publisher: { "@id": `${SITE_URL}/#organization` },
    // …
  };
}

export type Locale = "en" | "tr";
export const locales: Locale[] = ["en", "tr"];

export interface Dictionary {
  locale: Locale;
  nav: { services: string; work: string; studio: string; contact: string };
  // …
}

export const dictionaries: Record<Locale, Dictionary> = {
  en: { /* … */ },
  tr: { /* … */ },
};