It’s a regional dialect

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TL note: “Plats de poisson” is French for “fish dishes”.

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TL note: “Plats de viande” is French for “meat dishes”.

As a minor note, Poissonniere speaks in a very formal tone, as expected of a chef serving customers, but Patissciel speaks more informally, with a “DESU WA” pompous rich-girl tone.