As a translator I'm often thinking about idiomatic issues, especially the ones I believe are totally linked to culture , thus hard to translate.
In Brazilian Portuguese we have this "capricha", a demand when you are buying something to eat (on street stalls mainly). You ask the attendant to "caprichar" when you want them to give you extra topping, or extra sauce, or extra anything. It's not refill and you don't pay for it. It involves being friendly and getting some advantage because you're nice, and you ask the other person to be nice too.
Do you have any examples like this in your own languages?