the last two lines of /usr/local/psa/admin/plib/locales/ja-JP/messages_ja-JP.php are TYPO?
@include_once('common_messages_en-US.php');
@include_once('conhelp_en-US.php');
but,
@include_once('common_messages_ja-JP.php');
@include_once('conhelp_ja-JP.php');