Hreflang
Hreflang is an annotation that tells search engines which language and region a page targets, so the right version is shown to the right audience.
For sites with multiple language or regional versions, hreflang maps each version to a language code and optional region code. Done correctly, a searcher in Germany sees the German page and a searcher in the US sees the US page, without either being treated as duplicate content.
The requirement that trips most implementations is reciprocity: every version must reference all versions, including itself, and each must link back. Missing return tags or invalid codes cause search engines to silently ignore the annotations.
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