Hi there,
Unfortunately due to the nature of the fonts we can't support very many languages in this particular game right now, it's just limited to utf-8 and a fairly standard character set. Our later games use a different font rendering engine and I believe are able to support Chinese if someone extended the spritefont character set properly for it, but we've not had anyone try. AI War and Tidalis use an older version of our GUI system, though, and the way that the fonts are baked in makes it so that they are a lot less flexible than the later ones.
Sorry about that!