lang="" if you don't know what language your page will be in. <html lang="en">, and then <p lang="fr"> on whatever other language content. Content from users that aren't tagged to be in a specific language doesn't really fit into this system though.
https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...
That sounds good in theory. On bsky.social you are supposed to choose a lang before posting.
But again there's mixed language issue
Or do users even bother to choose the correct lang?