Filler gave you a good outline of the process, so let's look at how it applies to you.
Looking at a gameplay video on YouTube it looks like it uses just kana, which makes things easier. Basically, 8- and 16-bit consoles use one byte per character to save space, which limits you to 256 different characters. Obviously that's not enough for proper Japanese script that needs lots of kanji, and that would require two bytes (giving a total of 65,536, more than enough for every character imaginable). 32-bit and onwards systems generally use more accepted standards like Unicode or Shift JIS.
Anyway, the GBC is an 8-bit machine, but a very late one, so it can use huge cartridges compared to when the GB first came out. I translated the first Detective Conan, and it was clear that despite the amount of text, compression was unnecessary due to the huge ROM size. This game probably does the same.
However, I did a relative search using the kana order I found in the ROM, as well as using the typical kana order, and I found nothing. It's surprising, but you may need to use a different route to finding the text. On a NES it's easy because of how the system works, but GB is a bit trickier and I don't have experience with it.
It seems that unlike the NES, graphics are loaded into memory individually, then put into place in a different part of memory. The question is how they get into memory in the first place. I'm still figuring this out myself, hopefully I'll find something...