Unfortunately I don't see any way to use Japanese text in AppGameKit, it just isn't compatible.
AGK has been written to use ASCII only which is quite an antiquated method of using text. As the name suggests, It only really works for American text or non-accented English as I prefer to call it.
There are workarounds to use ASCII for accented characters and AppGameKit also uses a workaround with the ExtendedFontImage commands but even that is no use if you want to use the 'correct' method of encoding instead of ASCII.
So unless Paul adds support for unicode I don't think Japanese is an option.
There is of course the option of creating my own bitmap font system that supports unicode but I don't think I'll be doing that for this game. I'll probably just drop the idea of Japanese for now and look at it again when I've finished my current project
This article explains why ASCII has had its day and really shouldn't be used to reference characters.
AGK V2 user - Tier 1 (mostly)