Sticking to your native language for single-developer projects that consist of code you're not already comfortable with is pretty reasonable, especially if that language uses the Roman alphabet anyway. I'm not too inclined to look at code that's in Chinese though...
Sure, but we spanish people use "mañana" for saying "tomorrow". I find it odd to make a method
private string getTomorrow();
private string getManana();
Since the "ñ" isn't in the code language, also the mix of spanish-english "getManana" instead of "obtenerManana" ("get" means "obtener" in spanish). Anyway, if it works, it's ok anyway you do it, you can handle this sort of things later with refactor -> rename (Eclipse)