No, he just speaks four langauges, but Jill is a linguist.
A diagram of the fields of linguistics
Referring to a person of this nature as a Linguistic will drive them insane, as its not the correct use of the word. Also see Grammar Nazi.
john: Whats a restuarant? oh you mean restaurant. also I think you meant there not their.
tim: didn't realize that you were such a Linguistic.