Well... I'm not sure. I try to talk to them aaaaaaall the time and, I don't know why but, they don't talk back :-'( I have an idea... let's ask Whoopi Goldberg !!! She spoke with one once, remember? =p ^^ just kidding... yes, it is complicated !

the ghost does not exist, so there is no "ghost language". they are illusory in your mind, so the language that ghost speak is depend on you, you can choose any languages which you like.

I think they speak the same language that a person who believes in them speaks. :D

I don't think they will use languages.

Didn't you know that ghosts can speak any language they want to speak? And, by the way, how do you understand "ghost"?