From the site: "Sales of 'English Training DS' were approximately 242,000 units. Although there is a strong educational element to the game, sales have been strong from the very beginning, perhaps due to the large number of people who wish to learn English in a relaxed, informal way."
Hell, if someone were to release a Japanese teaching program for my DS that taught it in a "relaxed, informal way," I'd snap it up. I know a literature professor who knows nothing about video games who might consider buying it. Do you hear me, Nintendo? Make this happen please.
(Alas, they never listen to me. I think they screen their calls.)
I'm also interested in a private way to learn Japanese. I don't even understand which books might help me learn to read the Japanese that's in videogames...
The DS touch screen would be nice for Kanji training and practice. At least if it managed decent character recognition.