The same problem for me.
I posted my solution on http://wordpress.org/support/topic/344708?replies=7#post-1354001
But the solution is basically the same as Mr. koitaki’s one.
Same problem for me, too. I’ve solved the problem by doing this.
- Delete
wp-includes/pomo/translations.php
- Place new
translations.php which is newly downloaded wordpress-2.9-ja.zip
http://rozken.homelinux.net/wordpress
I tried wordpress-2.9-ja.zip because the version on the problem was it. Please check before you change the translation.php file.