1 - Yeah mainly English. Sometimes when you run with a PUG you’ll hear a different language but in my experience they were from other servers.
2 - My guild got AOTC for both alliance and horde (we’re dual factioned) 
In fact we are always looking for folks to join our ranks. We have a few different raid teams.
3 - It’s a mixed bag with so many people. You’ll find most are nice and helpful. However, there are some that aren’t.
4 - It’s a high population server. Sometimes it can make things like grinding out dailies interesting when you’re competing for the same resource but I haven’t had much trouble there. The auction house is lively.
If you come to Dalaran and are looking for a guild you can check us out:
Contacts:
Alliance side: contact a guild member via /who Pénsive (to get the accent press alt and type 130)
Horde side: contact a guild member via /who Pensive
Either side:
Co-GM Asch @ Bnet Aschien#11798
Co-GM DOC @ Bnet Brighthammer#1121642
Assistant GM Frahenstein @ Bnet Inzania#11304
GM Briar @ Bnet Briar#1421