I am not sure how it would be possible to block entire languages from being used on Discord. I am going to assume that you mean to block non-English alphabet, but there are plenty of other languages that use the English alphabet that we also don't allow spoken. And there are many overlaps between these languages, with some English words meaning something else in another language.
Furthermore, attempting to block it would probably lead to people thinking we are discriminating against their language, when what we are actually trying to do is enforce a common language that most people in the Discord know at least a little bit of, so that everyone can be included in the conversations and it's easier to moderate.
But on the topic of moderating it, this offence is not that serious anyways. That's why we always give verbal warnings for speaking in an unsupported language before even resorting to punishments like official (recorded) warnings or bans. Giving a verbal warning to explain the rule would actually, I think, be less confusing than having your message randomly blocked. Overall, I don't think this is something that should be filtered.