In /library/UnreadPostCount/Listener.php check the section
Code:
$unread = XenForo_Model::create('UnreadPostCount_Model_Unread')->getUnreadPostCount($userId, $nodeIds);
$whatsNewPhrase = new XenForo_Phrase('whats_new');
$whatsNewPhraseQuoted = preg_quote($whatsNewPhrase->__toString());
and try changing the segment XenForo_Phrase('whats_new') where the whats_new reflects the phrase in the language you selected (if the base ACP phrases are in another language - if in English it probably won't do any good).