Is there a ことわざ which means "don't talk about things you don't understand"?
There is commonly an used phrase which has the meaning as you say, it is 知ったかぶり but it isn't ことわざ.
In ことわざ, 知らざるを知らざると為せ是知るなり is the one but it isn't common. The meaning is that "知っていることと知らないことをはっきり区別し、知らないことは知らないと認めるのが本当に知ることだということ (Distinguishing what you know and you don't know, and it is truly knowing to accept what you don't know without pretending to know them.)"
Thanks, though does 知ったかぶり have the connotation that one is trying to pretend one knows more than one does? I was thinking about a situation where someone possibly doesn't even realize they don't understand what they are talking about---would 知ったかぶり be appropriate there?– KimballApr 4, 2016 at 12:49
Yes, it does. I think 知ったかぶり is appropriate for the meaning you said. Apr 4, 2016 at 14:06