Google's John Mueller recently confirmed again that Google Search does not use the HTML lang attribute. He said many screen readers do and Microsoft Bing might use it as well, but Google Search still does not.
This is a similar statement to what he said several years ago in a video response.
Here is where John said this recently on Twitter:
Google doesn't use these, but screen readers do. Also, i think Bing might use them.
— ⛰ johnmu is not a cat ⛰ (@JohnMu) October 6, 2022
Several years ago, John said "We don't use that at all. So we use the hreflang links if you have that if you have different language versions. But the language attribute within the HTML markup is something we don't use at all. We've found that this language markup is something that is almost always wrong. So we tend to ignore that."
Forum discussion at Twitter.
Note: This was pre-written and scheduled to be posted today, I am currently offline for Succos.