I tried Mint 13 when it first came out. And also had troubles with Japanese input -- so eventually went back to Windows, since I had to dual boot Windows anyway to get Japanese.
Possibly, they fixed it. But I see a lot of stuff on the internet, with frustrated users trying to use KDE as a multilingual OS. I'm a Canuck, living in Japan so I need both languages.
Pity....i really like KDE otherwise.
I am pretty certain that I am right, now, about the language localization issue, however. In which case, it doesn't matter what you do. The language won't be there. Of course, you can install Japanese as the default language and then use English as the interface. But in that case, the menus of applications will all be in Japanese, which can be a problem unless you read Japanese really well (I don't).