MovableType has a full Japanese interface and works great as a full-fledged CMS. I use it on a number of sites that use none of the blog features at all. MovableType has offices in Japan and local support if you need it.
MT released a new version last week that I haven't updated to yet, but the developers are saying that you can finally switch the language of the back-end interface. So, technically, you could develop your site with the back-end in English and yet allow content editors to choose a Japanese interface.
How does the Drupal interface handle modules on the back-end when you're using Japanese? A lot of the modules are developed in English and given their number I doubt all of the module Admin screens show up in Japanese. If your site Admin is comfortable with English you're OK, but I'd be interested to hear from someone with a working install.
You can add the text to be replaced quite easily using: Strings to search for: You enter the english expression, which will bring up the terms, then you can edit them in using the interface in both English and Japanese.
Easier to use than explain! Suggest you try an install, really quite simple to put up.