Apparently the phrase "pulling my leg" in English, meaning when someone is attempting to fool you, translates in Spanish as "pulling my hair."
Che told us about The Great Wall of Lucy Wu, in which people studied proverbs and acted out stories behind the proverbs. There were originally specific contextual meanings behind things like "eat in the east; sleep in the west." A story behind each one.
This reminded us, of course, of the Star Trek episode "Darmok" in which the alien language is entirely composed of opaque idioms and the literal is meaningless. We discussed that for a while, since it's one of my favorite episodes and I wrote a story using it as inspiration.
In a fictional world, you can make up your own expressions, or alter ones from the real world to fit. I mentioned two from my Varin world: "A rock roof is safer than a blanket," wrote itself but has no parallel in our world, while "out of the dark" is a variant on "out of the blue." A reader needs context in order to understand an idiomatic epxression.
If you run into a spot in your narrative where you instinctively want to put an English idiom, but it doesn't fit, think about what the secondary world context might support. Watch for contextual mismatches.
Morgan mentioned "feathers in the wind" as an idiom suggesting that gossip can't be captured.
Our capacity to understand metaphor helps us to understand idioms.
Che mentioned "blood adds sauce to the meal," which depending on context could mean "fresher is better" or "you have no taste."
My own character Rulii says things like "only the scavenger doesn't expect to find his meat still struggling," and "are your teeth sharp enough"?
Each idiom can have meta-implications.
Che mentioned how real world proverbs are often used to teach. "If you lie down with dogs, you wake up with fleas" "A bird in the hand is worth two in the bush."
You can have similar proverbs in your world.
Glenda talked about how you need to know the story behind "sour grapes" in order to grasp all the implications. This led us to talking about canonical stories. Are they religious stories? Folk tales?
If you are using idioms or proverbs in your world, you get more metaphor, and a bit of historical linguistics folded in. There's an opportunity there to have real events give rise to an expression that becomes idiomatically opaque later.
Che mentioned that if you are looking for idiom contexts, there are lists available on websites for people studying English as a Second Languge (ESL). Is there a character in your story who is a proverb-using person?
You can look for ideas in books of proverbs from other countries.
More detail and examples can be found in the video: