No big reason, the guys just post in their native language & don't bother translating. I've seen some Spanish here too from time to time.

You can highlight their text and translate with a browser extension (Mate Translate, DeepL, etc), or just be a baller and learn to read more languages! :)

Probably because English is not a universal language, and people sometimes write exclusively using their native/primary written language (or prefer to write in that language).

A great comment reply thread on this topic was from back in April. See the suggestion by nout to have SN only show posts and replies that are in the language(s) you specify in your settings (a feature that SN might have implemented at some point in the future).