DISCLAIMER: The above file is free for anyone to use, but realize that it is customized for specific needs of the site it resides on, does not strictly adhere to the default English meaning in all cases, and in some cases purposely blanks out messages. (It is by no means complete.) You would be well advised to use the one distributed with Hiroyuki Senshu's Win32 package if you need a ja.po file.
Henry Nelson
As of June, 2000, there is a Translation Project Japanese message file as well.