By all means, English should be the main and only language in the world or known by everyone and used. If you come to a server which is based in Europe and is multi cultural then yes, expect to speak only in one language. In this way we protect our players from being insulted in other languages.
If you want to talk in your language to someone, you have several options like: steam, skype, msn, ts3 etc. Anything what is not public.
If it's publicly then English is appreciated, if not, then don't say anything at all.
That's how it works.