Darn. Installed the update for Daigasso hoping I could local multiplayer with my Japanese 3DS afterwards. Doesn't find the local game from my Japanese 3DS, and when trying to create one itself, I get an error. Oh well.. :<
yeah that is a problem with running games region free. try running a different region of mario kart and trying to play local, i'm pretty sure it doesn't work.
but nsmb 2 has no problem like that. it's game by game basic you need to test.
i'm pretty sure you will be fine playing online together though.
(this has nothing to do with the update)