Ttaskmaster
Because it's big and clever and makes you sound cool?
Because we're English and sometimes have the need to offend foreigners in ways that they can actually understand?
No, no no. We're British. We've never had trouble offending foreigners, without bothering to actually speak their language. We just S A Y I T S L O W L Y and
V E R Y L O U D L Y in English, 'cos then, obviously, all foreigners understand us perfectly.
Nah, cussing someone a foreign language is best used on Brits,
because they typically don't speak the kanguage. That's why Arabic works well - so few (native) Brits speak it. By the time people work out that they've probably been insulted, conversation (and probably you) have long moved on.
Of course, there is a risk.
The above method is great for leaving people a bit confused and bemused, probably feeling they ought to be offended but not quite sure ….
unless you pick someone that happens to speak that language, in which case the above method is great for getting punched on the nose, kicked in the plums or in the case of one or two of those Arabic phrases, probably nose snd plums.
Russian works pretty well too. German isn't bad with all those gutteral and glottal sounds but has the singular disadvantage of being far too widely understood, thus presenting a serious risk to the health of the utterer's schnoz and nuts.
Personally, I think I need to come up with a few choice phrases maybe in Aramaic. Maybe a long-dead language …. anyone know Aztec? Perhaps Mayan?