@PixelGeek any update on this??
I have the same problem. I have added the javascript fix but in Ahrefs I get this error: Hreflang and HTML lang mismatch
Looks like Google’s first rendering now executes Javascript, we have performed a couple of tests modifying elements in the DOM with Javascript on page load, and confirmed that Google’s Crawled version of the page has the version modified using Javascript. You can use Google Search Console Inspect tool to see the crawled version of any URL (Indexed or not).
We haven’t test it out to change the “lang” attribute yet, but I think we will have to now 
, given that it appears there’s no custom setting at the page level to be able to do that.