Vanyel's first language is Cyrodiilic, with Nordic as a second and at least a smattering of the other common languages of Tamriel (and Dovahzul). He's easily nerd enough to have studied other languages (most likely Earth ones, given most of his Nexus acquaintances come from there) in his time in the Nexus, but probably not to the point of fluency.

So he will be basically incomprehensible to everyone, and vice versa. Have fun with that!

(Alternately, we could assume a degree of similarity between say, Cyrodiilic and Latin, or Skyrim-ese and Norwegian. Still not super helpful, considering how few other people speak them, but not quite complete gibberish either.)