Hreflang is a tag that tells a website that a given page has language or country variants, and which variant belongs to which audience. Thanks to it, the search engine displays the correct version and does not treat them as duplicate content. It matters most for linguistically close markets, where, without the tag, the Slovak and Czech versions compete with each other and the search engine shows the wrong one. The rules are strict: the links must be reciprocal, every version must link to all the others including itself, valid language and, where relevant, country codes must be used, and the URLs must be complete. Common mistakes include mixing up a country code for a language code, linking to redirected or non-existent URLs, and using hreflang as a substitute for an actual translation. It replaces the language of the content, not the currency or delivery.
See also: Website migration without losing rankings, Index bloat.
