project builder already comes localized in japanese.
and as far as header files are concerned -- well, yes, sure you could localize them to other languages. but one of the first rules non-native english speakers learn when beginning programming is to have their comments, variable and method names in english.
believe me, it makes sense. even when writing stuff together with other german friends, we always write comments in english. once it's become a habit, there's nothing weird to it.