Very good news for Italy: Mailplane now speaks Italian, too! Monica Cainarca did the job and she did it wonderfully. The translation is now available with the current stable release (1.61).
BTW: In case you have any English/Italian (or Italian/English) translation need, please check out Monica's McTranslation page. Not only does she general translations, but she knows how to do Mac software translations, too. It is a pleasure to work with her - I highly recommended her services.