I'm writing a story set (primarily) in modern-day London with a few elements of fantasy, but several of the characters are foreign, and most are multilingual; their profession is a sort of mix of researchers and secret agents, so they have to be able to communicate in a number of different places. Several of them habitually slip into languages other than English for endearments or to curse, though, and I looked around on translation sites but I really don't trust them, so I thought I'd try here.
In short, I need curse words and/or terms of endearment in any/all of the following languages: Irish Gaelic, Arabic, Romanian, Welsh, Russian, Croatian, Chinese (Mandarin), Greek. They're listed in order of importance, and sticking to the Roman alphabet renderings would be preferable.
If anyone knows a really good translation site (do they actually exist?) that I can go to for any of this in the future, that would also be lovely, but I'm not holding my breath.
EDIT: Thank you all very much! This was hugely helpful.