Reddit reviews Making Of Star Wars

According to one of these forum replies, George Lucas himself gave an answer to this issue while speaking in the voice of C-3PO.

> "There are literally hundreds of Ambassadors, to many systems, including several ambassadors from Alderaan. We would take ambassadors and important government people from Alderaan to all over. Mainly I'm an interpreter robot. If we found alien life-forms or if we were on a planet where they did not speak the Universal Galactic Language, I would have to translate. So I wasn't really in on what was going on. I was programmed to mention having no knowledge of the Princess - that she was aboard the ship. That's why I always seemed ignorant when she was brought up. I knew who she was, but I was programmed not to say who she was"

Apparently this book is the source of the quote.