hreflang

hreflang is an annotation — in HTML link elements, HTTP headers, or sitemaps — that tells search engines which language and regional variants of a page exist, so users get the right version. Implementations fail in predictable ways: annotations must be reciprocal (each version lists all others), self-referencing, use valid ISO language-region codes, and include an x-default fallback. It is among the most error-prone technical SEO items.