When a translator signs up using Special:TranslatorSignup, there should be a screen telling the user that the signup was successful. I would suggest something like this:
<big>Thank you for signing up.</big>
You will receive notifications when there is new material to translate. If you wish to change your settings later, visit [[Special:TranslatorSignup|the sign up page]] again.
On another semi-related note, it is not very intuitive how to unsubscribe. The way you do it is to uncheck the boxes regarding how you would like to be notified, but maybe there should be an unsubscribe button as well?