I can't stand people who can't say their opinion ...

[Anna232]

Does "say their opinion" sound idiomatic? Can it be used instead of "express"? And when talking about many people should it be "back" or "backs?"
Context: I am sharing my opinion about what annoys me in people.
I can't stand people who can't say their opinion to another person's face and people who talk about others behind their back(s).

 

[emsr2d2]

Does "say their opinion" sound idiomatic? Can it be used instead of "express"?

No. Use either "express their opinion" or "say what they really think".

And when talking about many people should it be "back" or "backs?"

You can use either. I generally use the singular.

Context: I am sharing my opinion about what annoys me in about people.

See above.

I can't stand people who can't don't say their opinion say what they really think to another person's face and people who talk about others behind their back(s).

See above.