Unfortunately, it doesn't exist (as far as I know).  This would be a very difficult program to write.  Accessing the text of a game is hard, and hackers have to usually treat each game differently and do a lot of research just to get the text from each game.  The only alternative is some kind of OCR-like character reading, which has its own challenges. 

The DS emulator doesn't know anything about text, it's just doing whatever the ROM tells it to do.  It can't tell the difference between displaying text and displaying any other image.  The ROM of course has sections that are coded to display text, but they're different for each program and the emulator doesn't really have any way of discovering when it's being asked to display text.

OCR seems like the only option.

You'd think though... that a Japanese character recognition app would exist that would work with any software, just grabbing the content of a window or area of the screen?

You could just buy a paper-printed japanese-english dictionary, you know. If you don't have one, you can also print JLPT vocab lists and use them as a small dictionary. Most words in that game are N2 or easier, though the legal terms are in the N1 list (fortunately, they're explained within the game).