Personally, I think it would be cool to hear characters speak in a native language like in Tekken. I get that is expensive, but imagine getting the choose a language for CAS as well, heehee, I sound like such a Fangirl/boy, but i'm literally shaking with excitement of the thought even though it's never going to be added :D
It's not like I can even understand most of them, but i'd still take Eng (because we can choose to have everyone be american 'cause DUBZ)
Japanese covers Maxi, Mitsu, Taki, Setsuka.
Maybe Chinese or mandarin (sorry, don't know the difference because i am an uncultured swine) for X, Kilik and Maxi since he usually follows X and Kilik around
Maybe like french (raph, Amyiola, And IVY (I imagine her speaking french, for some reason, and yes I realize she is british))
German (for Sieg and Hilde, Zwei, Viola, Tira and MAYBE nighty).
I would say Korean, but I doubt that we will have more than 1 of the koreans, probably Hwang, or Yun. Get Talim to speak korean as well if it's an option.
I feel like that would cover most characters that have chances of being in.