PortaBilling100 has an excellent multi-language support. The whole solution is supplied with the source code, and translation of the web interface is just editing of the XML file. So you can have Japanese or some other language (Chinese traditional and simplified are already there) in just a few days.
And because this so easy – PortaOne users do translation into new languages all the time (right now there are 13 languages including Arabic) – so you may even get Japanese for free if somebody else will do it before you.