Can't instances and communities set a default language for posts/comments, and are language tags available for everyone in the Threadiverse?

I have selected only undetermined and languages I speak in my profile, and it seems to have improved my feed a lot, however I had to block dozens of country/language-specific instances and communities because people there are not using language tags so their content keeps appearing to me... just wondering why instances in certain languages don't have their content defaulted to their language.

There's also this one Italian guy I had to block because he constantly posts in Italian on English-speaking communities on English-speaking instances and doesn't use the language tag... I only post in my language on instances/communities from my country, and even then I use the language tag so it doesn't leak into other people's feed (if they are not interested in seeing stuff in that language)... I don't know, it just seems kinda rude to not use language tags since they are available (but perhaps they aren't available to everyone in the Threadiverse?)