using different language in Dashboard and in the blog (Theme)

Hi everyone,
I have a blog I am trying to set up for Japanese readers. I would like to have my administrative pages (Dashboard) still in English, while the blog messages, date etc. itself Japanese (i.e. the reader of the blog can see the headers). I have translation for the theme, and I have put the language file under the wp_content/theme/themename/ but I have to set the wp_config default language to ‘ja’ in order to activate it.

So the problem is that how can I set up a mixed language environment: change the language of the theme without changing the default language.

Any hints are appreciated, as I am beginner in this.