admin: use English as default language
admin: add Japanese admin interface

It is very silly way to localize, but it is better than none at all.
Improving admin interface task is just began, so it
will be replaced by more sophisticated way in the future.
admin: add language switch links
Merge pull request #182 from kenhys/use-english-as-default-lauguage

Use English as default language

Patch by HAYASHI Kentaro. Thanks!!!