Even if the person you are mocking doesn't understand your language, humans are very easy to read based on tone and body language alone so just don't do it.

I personally love tras.h.i.+ng people in their own language if they ever try to mess with me and it is an absolutely wonderful feeling for me. And as an added bonus it must feel utterly depressing for my counterpart. Although it rarely ever happens and it's usually some foreigners mocking locals, I can't help wanting to teach them a lesson that they probably won't forget anytime soon.

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The number one rule of insulting someone is just don't do it since there shouldn't really ever be a reason to. It makes you appear as a petty shallow person especially if you don't have the b.a.l.l.s to confront the person.