Hreflang

An HTML attribute that tells search engines which language and region a page is intended for, linking to equivalent pages in other locales.

Hreflang (<link rel="alternate" hreflang="en-US" href="...">) solves the international duplicate content problem. Without it, Google may choose the wrong language version of your site for a given user. Hreflang annotations must be bidirectional — if your English page points to a Spanish equivalent, the Spanish page must return-point to the English page. Missing return links are one of the most common hreflang errors.

The value uses ISO 639-1 language codes (en, es, fr) optionally combined with ISO 3166-1 country codes (en-US, en-GB, fr-FR). Use "x-default" to specify the fallback page shown when no regional match exists. Hreflang can be implemented in the HTML head, HTTP headers, or XML sitemap.

Common errors: self-referencing annotations missing, non-canonical URLs used in hreflang, incorrect language/region code format, and pages that are hreflang-tagged but blocked by robots.txt (which prevents Google from reading the tags).