This may already be widely known, but I discovered this yesterday by accident while playing on my Japanese account.

If you subscribe to Nintendo Online you can get NES/SNES games and N64 if you pay more. But if you want to play in Japanese you need the Famicom apps.

Initially I thought you need to purchase the Japanese subscription as well, but if you log-in to a Japanese account you can download the Famicom apps. Then if you open the app on your main account, the one with the NSO subscription, you can access all the Famicom games in Japanese.

In addition to a decent amount of exclusive games like SMT and Fire Emblem that I guess aren't translated into English. I was really excited about this discovery and thought I'd share it with everyone on here who might also have a NSO subscription and didn't already know.