old/application/language/en/notification_lang.php

<?php
    $lang['users.newFollower.single'] = '%1$s is now following you.';
    $lang['users.newFollower.multiple'] = '%1$s and %2$s are now following you.';
    $lang['users.newFollower.many'] = '%1$s, %2$s and %3$s others are now following you.';