Hreflang
An HTML or XML sitemap annotation that tells search engines which language or regional version of a page should be shown to users.
Hreflang is an international SEO signal used to connect equivalent pages for different languages or regions. For example, an English page, Spanish page, and x-default page can reference each other with hreflang annotations so Google serves the right version to the right user.
Good hreflang implementations include a self-reference, reciprocal alternates, valid language-region codes, and an optional x-default URL for users who do not match a specific locale.