1. instead
"user_online_end" => ($user_online_list) ? ":" : ".",
find
$site_template->register_vars(array(
"num_total_online" => $num_total_online,
and insert above
$user_online_end = ($user_online_list) ? ":" : ".";
2. find
"lang_user_online_detail" => str_replace(array('{num_registered_online}','{num_invisible_online}','{num_guests_online}'), array($num_registered_online,$num_invisible_online,$num_guests_online), $lang['user_online_detail']),
replace
"lang_user_online_detail" => str_replace(array('{num_registered_online}','{num_invisible_online}','{num_guests_online}','{user_online_end}'), array($num_registered_online,$num_invisible_online,$num_guests_online,$user_online_end), $lang['user_online_detail']),