It's a tough call for me. I played FFXI for years so I'm use to seeing multiple languages in chat. I'm use to trying to communicate with the auto translator. At the same time I've played other MMOs like WoW, Aion, Rift, where everyone pretty much speaks English and it's nice to know when I talk to someone they'll understand me.

A simple solution would be to name servers certain languages but not have restrictions where someone goes. This way people who would rather be with like speaking people they could be while others who don't mind or speak more then one language has options.

And last, if servers was to be split up it's not completely bad. It would be a huge change but if servers was split up then merged so there was a decent population on a server of people speaking the same language you would notice a huge difference. Regardless if you don't mind playing with a wide variety of people speaking other languages, most likely everyone sticks with people who speak your language. Given a server that is mostly the same language it will feel more populated.