I know, it's always a tricky subject to implement.
Also I'm not an expert, so maybe my understanding, explanation and proposal make no sense. 😉
But here how I see it.
1st step
could be to allow the user to change the lang attribute at the HTML level.
Which would allow user to publish websites in others languages than english without hurting their SEO.
2nd phase
would be to allow to control the language per folder, such as if I make a folder fr and define the language at the level then the pages and folders under that would have lang="fr" as global attributes
https://www.w3schools.com/tags/att_global_lang.aspWhat do you think.