The way I was thinking about this is that rather than sub it could be more useful if you can create posts in different languages and then on homepage you would only see posts in your selected languages (e.g. in english + spanish).

By default you would only see english posts (so the same as currently), but if you add more languages in your config page, you would see those too (if you have more than 1 language, then the selection flags could show up in the "create new post" page).

Having sub for specific language is also possible, but that's a different usecase - in ~spanish sub I'd expect conversation about "spanish" things (rather than "bitcoin conversation in spanish language")

That said this idea isn't fully baked - it will require some more thinking :)