I always put it in English because the one time I 
did it in Kanji, it took about a month to arrive. 

I think you can put english and kanji on the 
front as well.

I don't think there is any rule against not using 
english, but I would double check the postal 
website (www.usps.gov).  You have to remember 
that it is going to go through a lot of different 
(english reading) hands before it goes to Japan, 
so make sure at least Japan is written in English.