I'll swear in Italian, but that's about as strange as I get (I'm trying to expand that to French, German, and Spanish, but I haven't yet - except for the German, which is too obvious to use in public anyway).
I swear in Italian too, but that's not a big feat for me.

Seriously, I swear in other languages too. Nothing too fancy (English, German, Greek, a bit of Japanese). My favourite are German scheisse, Japanese kuso and baka, Greek malaka. Spanish swear words are fun, but too similar to Italian for me to use in public.
I'm not too fond of invented swear words. For some reason, I can't use them seriously.