Subscribe2 attempts to check that emails have been successfully sent by reporting the callback returned by the WordPress wp_mail() function.
In your server or WordPress install are not correctly configured this can result in error message despite successful sending of emails.
To overcome these issues access the PHPMailer files directly, locate a language error file and uploaded to your WordPress site (wp-content/languages/ for example).
Next, get yourself a Mailer plugin like the one suggested here and put the following line in the code.
$phpmailer->setLanguage( 'en', '../wp-content/languages/' );
This will ensure that you don’t get:
Language string failed to load: instantiate