Hreflang
An HTML or HTTP annotation that identifies language or regional variants of a page to search engines.
For businesses serving several countries or languages, hreflang helps search engines show the right local page, which keeps prices, currency and legal terms correct for each market and reduces duplicate-content confusion.
A retailer with en-us, en-gb and de-de versions of each product page adds a full set of link rel="alternate" hreflang tags on every version, including a self-reference and an x-default pointing to the country selector.
Hreflang is a hint, not a directive, and is ignored when return links are missing, codes are invalid such as en-uk, or targets are redirected, noindexed or non-canonical. It does not translate content or improve rankings on its own.
Each localized URL lists itself and every alternate with a language code in ISO 639-1 format, optionally followed by a region code in ISO 3166-1 alpha-2 format, such as en-gb. The annotations can be placed as link elements in the HTML head, as HTTP Link headers for non-HTML files, or in an XML sitemap using xhtml:link entries. Annotations must be reciprocal, and x-default designates the fallback page for unmatched users.
Systems Architecture
Google Search Central — Localized Versions of your Pages — https://developers.google.com/search/docs/specialty/international/localized-versions; WHATWG — HTML Standard: Links — https://html.spec.whatwg.org/multipage/links.html