Next.jsにおけるSEOとi18n：翻訳だけでは不十分

export const locales = ["en", "fr"] as const;
export type Locale = (typeof locales)[number];
export const defaultLocale: Locale = "en";

/** デフォルトロケールでない場合はロケールをパスの前に付ける */
export function localizedPath(locale: string, path: string) {
  return locale === defaultLocale ? path : `/${locale}${path}`;
}

/** 絶対URLヘルパー */
const ORIGIN = "https://example.com";
export function abs(locale: string, path: string) {
  return `${ORIGIN}${localizedPath(locale, path)}`;
}

import type { Metadata } from "next";
import { locales, defaultLocale, localizedPath } from "@/i18n.config";

export async function generateMetadata({
  params,
}: {
  params: { locale: string };
}): Promise<Metadata> {
  const { locale } = params;

  // 正しいJSONファイルを動的にインポートする
  const messages = (await import(`@/../public/locales/${locale}/about.json`))
    .default;

  const languages = Object.fromEntries(
    locales.map((l) => [l, localizedPath(l, "/about")])
  );

  return {
    title: messages.title,
    description: messages.description,
    alternates: {
      canonical: localizedPath(locale, "/about"),
      languages: { ...languages, "x-default": "/about" },
    },
  };
}

export default async function AboutPage() {
  return <h1>About</h1>;
}

import type { MetadataRoute } from "next";
import { locales, defaultLocale, abs } from "@/i18n.config";

export default function sitemap(): MetadataRoute.Sitemap {
  const languages = Object.fromEntries(
    locales.map((l) => [l, abs(l, "/about")])
  );
  return [
    {
      url: abs(defaultLocale, "/about"),
      lastModified: new Date(),
      changeFrequency: "monthly",
      priority: 0.7,
      alternates: { languages },
    },
  ];
}

import type { MetadataRoute } from "next";
import { locales, defaultLocale, localizedPath } from "@/i18n.config";

const ORIGIN = "https://example.com";

const expandAllLocales = (path: string) => [
  localizedPath(defaultLocale, path),
  ...locales
    .filter((l) => l !== defaultLocale)
    .map((l) => localizedPath(l, path)),
];

export default function robots(): MetadataRoute.Robots {
  const disallow = [
    ...expandAllLocales("/dashboard"),
    ...expandAllLocales("/admin"),
  ];

  return {
    rules: { userAgent: "*", allow: ["/"], disallow },
    host: ORIGIN,
    sitemap: `${ORIGIN}/sitemap.xml`,
  };
}

import { getIntlayer, getMultilingualUrls } from "intlayer";
import type { Metadata } from "next";
import type { LocalPromiseParams } from "next-intlayer";

export const generateMetadata = async ({
  params,
}: LocalPromiseParams): Promise<Metadata> => {
  const { locale } = await params;

  const metadata = getIntlayer("page-metadata", locale);

  /**
   * 各ロケールのすべてのURLを含むオブジェクトを生成します。
   *
   * 例:
   * ```ts
   *  getMultilingualUrls('/about');
   *
   *  // 戻り値
   *  // {
   *  //   en: '/about',
   *  //   fr: '/fr/about',
   *  //   es: '/es/about',
   *  // }
   * ```
   */
  const multilingualUrls = getMultilingualUrls("/about");

  return {
    ...metadata,
    alternates: {
      canonical: multilingualUrls[locale as keyof typeof multilingualUrls],
      languages: { ...multilingualUrls, "x-default": "/about" },
    },
  };
};

// ... ページの残りのコード

import { getMultilingualUrls } from "intlayer";
import type { MetadataRoute } from "next";

const sitemap = (): MetadataRoute.Sitemap => [
  {
    url: "https://example.com/about",
    alternates: {
      languages: { ...getMultilingualUrls("https://example.com/about") },
    },
  },
];

import { getMultilingualUrls } from "intlayer";
import type { MetadataRoute } from "next";

// すべての多言語URLを取得する関数
const getAllMultilingualUrls = (urls: string[]) =>
  urls.flatMap((url) => Object.values(getMultilingualUrls(url)) as string[]);

// robots.txtの設定を生成
const robots = (): MetadataRoute.Robots => ({
  rules: {
    userAgent: "*", // すべてのユーザーエージェントに適用
    allow: ["/"], // ルートパスは許可
    disallow: getAllMultilingualUrls(["/dashboard"]), // /dashboard以下の多言語URLはクロール禁止
  },
  host: "https://example.com", // ホスト名
  sitemap: `https://example.com/sitemap.xml`, // サイトマップのURL
});

export default robots;