Hreflang

Hreflang annotations link the language and country versions of a page to each other.

Hreflang links outside the head: The page’s hreflang links end up in the body of the HTML, where Google does not read them, so its language versions are not connected.
Hreflang set without an x-default: The page lists several language versions with hreflang but no x-default, so visitors who match none of them have no chosen fallback page.
Invalid hreflang language or region code: One or more hreflang links use a value that is not a valid language or country code, such as en-UK, so that entry cannot target its audience.
Same hreflang value used more than once: The page lists the same hreflang value in more than one link, often for different URLs, so search engines cannot tell which version to show.