Earlier today reports coming from the Nikkei were suggesting that BlackBerry had decided to stop selling handsets in Japan in part due to declining sales and costs associated with making the necessary language modifications to the OS. It's not overly surprising this would be the case because if there is no money to be made then there is no reason to cater to that market. To help further clear things up over at Engadget they've received further clarification on the matter, which you can read below:
We are in the process of launching BlackBerry 10 globally in key markets and we are seeing positive demand for the BlackBerry Z10 in countries where it has already launched. Japan is not a major market for BlackBerry and we have no plans to launch BlackBerry 10 devices there at this time. However, we will continue to support BlackBerry customers in Japan.
So there you have it. If you want a BlackBerry 10 device in Japan, you'll have to look toward importing one. But if you're an already existing BlackBerry OS user and in Japan your device will continue to be supported. The sales issues is more at hand here and not the supporting of the Japanese language. BlackBerry 10 supports 33 different languages in total including Japanese.