Hi,
The correct code should actually be ja_JP.
It is built up by <language>_<country>. where country is used for
localization and language for translation if I recall correctly.
Don't remember where the official codes and descriptions are, but it
should be possible to find it with google.
Rails should fall back on language if the full locale is not
available, so it is either ja or ja_JP where ja is used if ja_JP
doesn't exist (or directly specified of course)
/Jimmy