hreflang
The hreflang attribute is an HTML or HTTP header declaration that website operators use to mark language-specific or country-specific variants of a page, so that search engines show users the right version. Google uses this declaration to serve the correct language version in the search results depending on the user's location and language settings.
In practice
It is important that hreflang declarations are always reciprocal: every language version must reference all the other versions, including itself. A common mistake is to set the tag in one direction only, which stops Google recognising the relationship reliably. An x-default tag is also advisable as a fallback version for users who match none of the defined language or country variants.