I used to search like mad for games to learn Japanese. I eventually gave up. Here are the two points as to why:
1) Just as a matter of the time it takes to learn, flash cards (e.g. Anki, etc) are a much more efficient way to learn vocabulary (no time wasted walking around a slime forest).
2) If you want to "immerse" yourself in something fun to learn Japanese, a good book provides infinitely more depth and breadth of Japanese language experience than a video game, so rather than playing games, reading is probably a more effective tool for language study.
However, when video games might be helpful is if you cannot make yourself sit down and open your flashcards or crack open a book, it might give you some Japanese practice you might not have gotten otherwise.
But so far, the games that teach Japanese aren't really even worth your time. I've tried them all:
slime forest
http://lrnj.com/,
japanese kanji game pack
http://www.download.com/Japanese-Kanji- … l?hhTest=1
Knuckles in China Land
http://www.kicl.info/
JRPG
http://chaosforge.org/taw/jrpg/
Kanji trainer penpen
http://www.coolest.com/penpen/
(also some teach to type programs, Nintendo DS games - not the Japanese flash card type - flash cards are more helpful - but actually games like the Kanji Kung Fu ones).
I would actually say the best of the bunch is not Knuckles or Slime Forest but JRPG.
Maybe it would be different if someone actually made a fun game that could really make kanji or vocabulary stick in a way that was superior to just good old fashioned flash cards or books, but so far none meet the mark, so I recommend either developing your own game and sharing with the rest of us (come on, there has to be some budding genius programmer who is also studying Japanese) or using the tried and true methods.
My 3 cents.