What triggers this issue?
This issue reports page URLs where the language (or language-location) code declared in the hreflang annotation is invalid. Language code must adhere to ISO 639-1 format, region code must be specified in ISO 3166-1 Alpha 2 format.
Why is it important?
Google, as well as other search engines, will ignore hreflang annotations with invalid language (or language-region) codes. Consequently, search engines may "overlook" alternate versions of your page and will not be able to point users to the most appropriate version of your page by language or region.
How to fix it?
For each reported URL, look at the columns shown in the screenshot below:
Look for "No" in the "Is valid hreflang" column to find invalid values in hreflang annotations.
"bel" in the example above is not a valid language code for Belarusian and must be replaced with "be."
If you need a list of URLs that have a given invalid language or language-region code, you can easily do it with one additional filter.
Make necessary edits to the hreflang annotations on all reported pages.
Multiple language/locale versions of a page can be specified in:
1. HTML tags
<link rel="alternate" hreflang="de" href="https://ahrefs.com/de/" />
<link rel="alternate" hreflang="es-es" href="https://ahrefs.com/es/" />
2. HTTP Headers Link
<https://ahrefs.com/de/>; rel="alternate"; hreflang="de", <https://ahrefs.com/es/>; rel="alternate"; hreflang="es-es", ...