Have you tried it?

The last time I tried was back on 12.04 so things may have improved significantly, but these are the main problems that I found with trying to use Linux in Japanese (in order of the problems I ran into):

1. Language translations are sometimes a bit iffy. The common stuff seems fine, but once you started installing more software, you realize that the software ecosystem just isn't there yet.
2. Fonts. No easy way to know which fonts to install that look good throughout the GUI. Then you also have to know how to configure the fonts. Oh and if you are like most office workers, you need a lot of font options for working with office docs. I mean, do you write everything in Times New Roman?
3. Keyboard input. Plug in a Japanese keyboard and hit the input switch key. Does it work? Of course not because installing Language Support doesn't setup the IME. iBus is a lot better now (I use Fcitx though), but at the time I tried, it was undergoing a pretty big transition and was a complete mess. 3b. Got your IME working? Now try it in all your programs. Unless your distro does it for you, chances are you missed a configuration somewhere and it won't work in all your programs because GTK2, 3, QT, etc. all handle the input separately. Now have fun figuring out what wasn't setup for you.

If you can get past all of that, you will have a working Japanese OS with random English and a GUI that doesn't seem like it's sized (fonts too) right for the language cause it's stuck in no-mans land between English and Japanese. Now you have to learn a completely new UX paradigm (Unity) and shortcuts without helpful hints like in Windows (this is what happens when you hit Alt in MS Word). Oh, and this is all until fine until you have to do it again on a different computer or if your upgrade somehow borks between points releases.

I don't mean to sound too down on Linux, but we are ridiculously far behind here. Without Kylin and Deepin fixing things in Linux for Asian users, we'd probably have zero adoption in that region.