Y revisastes la documentación oficial de esa classe?
Fijate el comentario:
http://phpmailer.sourceforge.net (en su página de inicio .. noticias):
Cita: Multiple Languages
PHPMailer now supports error messages of multiple languages. In the language directory you will find several files (only two actually, if someone would like to contribute more translations then please do so) like phpmailer.lang-en.php. If you look right before the .php extension there are two letters. These represent the language type of the translation file. For instance "en" is the English file and "br" is the Portuguese file. Chose the file that best fits with your language and place it in the PHP include path. If your language is English then you have nothing more to do. If it is a different language then you must point PHPMailer to the correct translation. To do this, call the PHPMailer SetLanguage method like so:
// To load the Portuguese version
$mail->SetLanguage("br", "/optional/path/to/language/directory/");
Un saludo,