I have a 2009 Mazda Axela which comes with a navigation and stereo system that is in Japanese. Is it possible to change the language to English?
You might want to try if your メニュー option (Menu) has a 設定 (Settings) option and inside there, a 言語 (Language) option. If it does, then most likely you can switch between the default 日本語 (Japanese) and 英語 (English).
If you don't find it there but you own the car, it could be a matter of researching what Pioneer unit you have and get an update, but chances are you have a domestic unit that hasn't been localized into English. If you own the car it might be worthwile bringing it to a Mazda shop, they will help you out.
Hope it helped!