This should be trivial, execpt for one thing, namely displaying the fonts in the game, does CEGUI support unicode fonts? If that is the case then we should be good. At least I cannot see other problems than that, how about you?
Edit: a quick google gave this
, which clearly says that CEGUI supports unicode, so we should be fine. But it still might be a good idea to use GNU gettext, to standardize how we implement the muli-lang support.