What triggers the issue?
This issue reports pages declared as alternate language versions that don't list themselves in their hreflang annotations.
Example:
Hreflang annotations for https://ahrefs.com/blog/zh/website-audit/:
<link rel="alternate" hreflang="x-default" href="https://ahrefs.com/blog/website-audit/" />
<link rel="alternate" hreflang="en" href="https://ahrefs.com/blog/website-audit/" />
<link rel="alternate" hreflang="ru" href="https://ahrefs.com/blog/ru/audit-saita/" />
The page must reference itself in its hreflang annotations. The following line is missing:
<link rel="alternate" hreflang="zh-CN" href="https://ahrefs.com/blog/zh/website-audit/" />
Why is important?
The rel="alternate" hreflang="..." tag helps manage multilingual websites by allowing you to specify language- or region-specific versions of a page. Based on this information, search engines can serve the relevant version to the users based on their language (and alternatively location).
All language variations of the same page need to be interlinked via hreflang annotations. In plain English, if you have About Us page in 3 different languages, each of these 3 pages need to link to the other 2 versions as well as have a self-referencing hreflang link. If the latter is missing, it can confuse the search engines and hreflang annotations can be ignored.
How to fix it?
You can manage your hreflang annotations via HTML tag in page header:
<head>
...
<link rel="alternate" hreflang="en" href="https://ahrefs.com/blog/google-keyword-planner/" />
<link rel="alternate" hreflang="de" href="https://ahrefs.com/blog/de/google-keyword-planner/" />
...
</head>
Or place them in HTTP headers:
Link: <https://ahrefs.com/blog/google-keyword-planner/>; rel="alternate"; hreflang="en",
<https://ahrefs.com/blog/de/google-keyword-planner/>; rel="alternate"; hreflang="de"
Whichever method you are using you need to make sure each language- or region-specific page has a self-referencing annotation. For example, a Spanish version of your page must have rel="alternate" hreflang="es" annotation with a link to itself.