Hello there!
I am working as a developer and I support a customer who chose to use the Newsletter plugin. The customer wants to adjust the wording of the plaintext versions of  confirmation and confirmed messages sent by the system. And I found this function in the 
subscription/subscription.php as the source of the wording … /**
     * @todo Move texts in the _get_default_text() method
     */
function get_text_message($type) {
        switch ($type) {
            case 'confirmation':
                return __('To confirm your subscription follow the link below.', 'newsletter') . "\n\n{subscription_confirm_url}";
            case 'confirmed':
                return __('Your subscription has been confirmed.', 'newsletter');
        }
        return '';
    }
… can we hope for someone to do the todo anytime soon, so we can adjust the wording? Adding a filter here would be great. The only other option I see to adjust the wording would be to hook into the __() which is not a great way …