A Japanese WordPress user has just released a multilingual admin plugin for WordPress, called “tam-mothertongue“.
One of the great thing about WordPress is that you can have multiple users writing and contributing to a single blog. By using this plugin, each user can choose their locale language for the admin screens.
For example, if you run a blog on the football World Cup with multiple users from various countries contributing to the blog posts, each users will be able to choose the admin screen language from Japanese, English, Spanish, French, etc, etc. (You will need the .mo files for each languages)
Of course, if the outputs language of the all posts are in say English, I am sure all the users will have no problem finding you way through the English language admin. But if thats not the case, this might be the plugin you are just looking for.