Requiesce in pace is Latin for “Rest in Peace”. It’s not Italian like many think. The reason Ezio may have used it instead of the Italian Riposa in pace, may have been because Roman Catholicism was the religion in the Renaissance (and in Italy) and the language used in Catholic rites, at that time, was Latin.
Now, as for why Arno is using it, possibly because it was commonly used in 18th century France. Paris, were Unity is set, has the Quartier Latin (Latin Quarter), were many Universities are located and were Latin was taught since the Middle Ages. Learning Latin has always been considered a high standard, specially for scholars and families with important tittles. It would be no surprise Arno was taught Latin either by the De la Serre family or the Assassins.
~Claudia (History enthusiast)