Hm, I've seen that there are patches already made for most of them, but it looks like Pichu Bros hasn't yet been added to the bottom of the list over here
, so perhaps it's still a work in progress. Those patches are unofficial, so they're not denoted on this page
, which lists all of the games which have officially been released by Nintendo.
There's a decent amount of homebrew out there, but I haven't heard of any that's Japanese-only. I'm pretty new here myself, so hopefully someone else more knowledgeable than myself can chime in.
I'm mainly here to pitch in with some pixel art, so I've not looked into translations and patches much as of yet. I'm sure your help would be very much appreciated, and I for one welcome you to the board.
I just got my first mini in the mail this week and am super impressed with this little piece of tech history, so I'm glad to see others who are interested in keeping it active!