A hreflang tag tells search engines which language and, optionally, which region a specific page is written for, and points to the equivalent page in other languages or regions so search engines can serve the right version to the right searcher. The format pairs a language code with an optional country code, "de-ES" for German-language content aimed at Spain, distinct from Spanish content aimed at Spain ("es-ES") or German content aimed at Germany ("de-DE").
Every page in a set of language or region alternates needs to reference all the others, including itself, which means the tags have to be fully reciprocal: if the English page points to the Turkish version, the Turkish version needs to point back to the English one and to every other language in the set. A missing or one-directional hreflang tag is one of the more common international SEO mistakes, and it tends to fail quietly, showing the wrong language version to searchers rather than throwing any kind of visible error, which is why validating the full set rather than spot-checking one direction matters.
