That's a good question.
I actually found a few users when I put "Japan" in our search box. They asked questions about their plants in Japan.
People don't have to list their location in their profile, so it's possible there are more and we just don't know it.
We also have a Japanese Language SE, which I didn't realize until you posted this! You could pop into their chat room and invite them to contribute over here. They might not even know about us. That could increase our user base, and introduce more types of plants for us to learn about.
You'll need to join the site though, or else you won't have enough rep to go to their chat room. Because you have over 200 rep here, you automatically get 100 at every other one of your sites. That's explained here.
I hope you find some people!