Good solution.
However, event-notify.php may be changed in the future.
It would be safer to override qa_send_notification() function with your new plugin.
http://docs.question2answer.org/plugins/overrides/
For example:
function qa_send_notification($userid, $email, $handle, $subject, $body, $subs, $html = false) {
if($subject == qa_lang('emails/q_posted_subject')) {
$subject = '^site_title har fået et nyt indlæg i ^q_categoryname';
...
$subs['^q_categoryname'] = $catname;
}
return qa_send_notification_base($userid, $email, $handle, $subject, $body, $subs, $html);
}