There is already some posting in Japanese, and that shows that we have some Japanese speaking users, which is fine in itself. However, I think that people who are comfortable writing in English should prefer to do so, because it lets more people join in the conversation.
If suddenly there were a disproportionate number of threads in Japanese, it would bump threads off of the front pages which the majority of our users can participate in, and replace them with threads that only a small minority of our users can participate in, and that could be a problem.
In Sushichan a while back, there was an incident where suddenly one or two people started making a large number of new threads in Russian over a short time span, and I deleted the threads because I perceived it as a troll rather than a genuine attempt at having conversation. It was an uncomfortable decision, and made me feel a little racist. Suddenly having a huge number of Japanese threads happen would be a strange situation for me and the site, I'm sure, and I don't know exactly what I would really do. Since the Japanese language is so intertwined with our fanbase for obvious reasons, I would probably allow it, unless it became obvious that people were doing it to be intentionally disruptive to our English speaking majority, like I suspect with the Sushichan incident.
Edit: Basically, some threads in other languages are fine, because I am sure we have Japanese (and other) users who are not comfortable with English. However, it could be a problem if a minority language greatly displaces the majority language in discussion here.