"Ask them what they are suggesting that you do instead" I think that this is a key point that should be given more emphasis. If there are reasonable adjustments that OP can make to assist students who are less fluent in English, then I would argue that OP should consider making them. But it sounds like OP is already doing that by sticking to simple language. I don't think it's reasonable to expect an English-language course to entirely avoid the English language.

Yeah, I graduated from uni three years ago, but I definitely would not have followed this advice.

This sounds more like advice to OP's students than to OP themselves. OP wanted to know how to deal with students who haven't taken this advice, and have e-mailed them with hostility instead of genuine curiosity and a desire to learn.

@Buffy - I imagine because all published research must go through ethical vetting processes, otherwise it's not research, it's a random guy with a clipboard. I'm fine with not publishing it, I'm just wondering what are the problems with being a random guy with a clipboard.

Interesting, so in other words you can just omit the country when it's obviously a US state?

MHRA Author-Date, yeah