Here is my personal opinion on the subject.
You should only mimic positive expressions when speaking to someone. The reason is that if they already have negative expressions and you have some too, you are likely to get in a darker and darker mood. But as long as you are subtle enough you can mimic positive expressions because they will likely create a trusting mood and complicity.
Do you speak several languages ? If so, you surely have been taught/told that you don't need to understand all the words to get the global meaning. Sometimes you skip a word and you catch the meaning with the following sentences. I believe that with body language you can do it too.
If you don't understand a specific facial expression, you may pick some others signs. You can even ask the person to rephrase his/her answer, or ask a linked question to see if the reaction is the same, perhaps the expression will be slightly different and you will understand it.
I would then reproduce the expression when I can be alone in front of a mirror so that the next time I would understand it.
If you still want to mimic the expression I see 3 options :
- flash it really quick so that the other doesn't have time to pick on it, can be risky though and you need to do it at the exact same time otherwise it would be even weirder if the other person picks it up.
- Chances are that the expression is kind of funny, if you are with someone you can have fun with, just say something like "you made that weird face it looked like that, it was funny, what does it mean ?"
- find a reason to look elsewhere / hide you face (seek something in your bag, look around for a waiter, go to the bathroom, lace your shoe...)
That's how I feel about it, but maybe Vanessa will be able to have a more detailed or scientific answer for you. That's an interesting question and I'd clearly like some other opinions on it too.