Asking someone to speak in plain Japanese is not rude if it's done nicely. However, asking someone to speak in Teineigo seems strange, I would say. Because it sounds like a challenge.
Keigo(敬語) is composed of Sonkeigo(尊敬語), Kenjogo(謙譲語) and Teineigo(丁寧語). So, a Teineigo-only conversation sounds like a sorting Keigo quiz or something.
How about asking them like this?
These sentences are polite and don't specify what kind of speech to use. So they might reply in plain casual Japanese, or probably in very simple Keigo.
I think they, waiters/waitresses or some other kind of staff, want to communicate with you to do their jobs. It's nice to tell them that you know basic Japanese words by asking them to speak in plain Japanese. I guess that they will be happy to use simple Japanese phrases for you, because it's much easier for them to speak Japanese than to speak other languages such as English, Chinese, French, etc.