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If someone called attention to it, like "Fries? What are fries? We only sell chips here." then it might be Separated by a Common Language.
If it passes by without comment, then that's a much tougher call in terms of tropes. If there is no British English used anywhere, you might get away with calling it Translation Convention (they're speaking British English, we hear it as American English). There's also the possibility of Viewers Are Morons (if the executives in charge believed the target audience could not understand that the British call that particular food "chips").
For that particular example, there's We All Live in America
It also makes a difference if she was ordering what some British people/restaurants call fries (skinny fast-food-style fries) as opposed to what you'd get with fish and chips, which are more toward what we call steak fries or home fries in the US.
^ They never show us on screen what she got because the plot moves on from there, so I couldn't tell you. It does pass by without comment though.
We All Live in America might count.
Current Project: The TeamI don't know a lot about the show, but could she have picked it up from the American in the cast?
Edited by Unsung
I've been thinking tonight about one moment in House of Anubis in which Patricia, a British character who lives in England where the show is taking place, asked for fries at a restaurant and not chips, most likely so the American fans would understand it.
Is there a trope for this sort of thing?