What triggers this issue?
This issue reports page URLs where the language (or language-location) code declared in the HTML lang tag (<html lang="...">) is invalid. Language code must adhere to ISO 639-1 format, region code (optional) must be specified in ISO 3166-1 Alpha 2 format.
Why is it important?
Although Google might not be using the HTML lang attribute today, other search engines and programs, such as screen readers, do to understand the language of the page.
How to fix it?
For each reported URL, look at the columns shown in the screenshot below:
"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 all URLs that have a given invalid language or language code, you can easily do it with one additional filter.
Make necessary edits to the HTML lang tag on all reported pages.