E
Etaoin Shrdlu
Guest
Mutatis mutandis, because there's no other way of communicating the concept in any language I know of without laborious periphrasis. I use O sancta simplicitas! on the internet quite a bit, but don't think I would say it in conversation. And QED, of course, if that counts, which is may not, given that it's probably used by millions of people who don't actually know what the letters stand for, let alone the rest of the Latin language.