Hello.Japanese input is not supported, that's why BB10 devices aren't being sold in Japan.
http://supportforums.blackberry.com/t5/BlackBerry-Z10/Why-does-Blackberry-keep-insisting-on-not-prov...

http://supportforums.blackberry.com/t5/BlackBerry-Q10/Japanese-input-in-Q10/td-p/2358835