Scheiße, putain, maledizione and friends
I'm sure we've all had experiences with people asking us what a certain swear word would be in our native tongues or been sworn at in an alien manner. I, for one, have had my cat insulted, my servant's mother insulted, my servant's cat insulted, and a quite puzzling insult seemingly directed at a maternal bovines in general (which I just totally didn't get). And all this from just the one phrase (which won't be written in this post).
Anyway, onto my point (such a rarity). I have always thought the point of swearing was to convey a message of anger and ill-will towards your victim. So, what use is cursing someone when they don't understand the full magnitude of your utterance? So you call down fiery brimstone upon someone in Mongolian. All you would get in reply would be a blank expression, unless of course the cursee happened to understand Mongolian (which would probably result in a totally different scenario).
If the intention was to mask your insult, there are much better ways I would imagine. A crude example would be to call someone a "modern Oedipus". The beauty of such insults is that sometimes the recipient may take the statement as a compliment, and you can savour the irony of it all.
A possible exchange could look like this (just because I'm bored):
Person 1: I'm making you perform this demeaning task because I can.
Person 2: You're a modern Oedipus.
Person 1: Er...thanks!
Person 2: You fucking idiot.