When our kids were old enough, starting around 8 I guess, we began having them order for themselves in restaurants. We discussed what they would order (and how to do so) before the server came to take the order. Usually, the server looked at us and ignored the kids.. but it only takes a second and they direct the questions to the kids.
It's a restaurant, so it can be noisy. The kids had to learn how to speak clearly and loudly enough to be heard, how to wait their turn to speak, the ebb and flow of the "conversation". We STILL discuss what they will order, but we don't really have to discuss HOW to order anymore. This was great for our youngest, who is the more socially reserved of the two. She always has a "special" request at meals, lol. (she doesn't like any kinds of sauce, so for example, if we eat BarBQ, and she orders the kids rib plate, she requests they put no sauce). This really was a confidence booster for her, to have to add information when the server expected the order was done...
:)
I think part of the reason kids have a difficult time speaking to adults, especially ones they don't know, is because adults are authority figures for the most part. It is intimidating. Our kids take martial arts, and it has done WONDERS with their comfort level of conversing with adults, as all the instructors essentially are adults. Yet they model respect for others, including calling the students Mr. or Ms. first name.