Japanese is actually very easy to program in GameMaker, it just takes a little more work. For starters, you'd be working with a sprite font (more popularly known as a bitmap font I think). You would need a script, which is easy enough, and then a series of fonts to go back and forth between since the syllabary is so big. More fonts would be required for kanji. It'd be a lot of work, but it'd be doable.