Anyone know if this is borrowed from Italian 'pranzo', or a cognate of it?
Conform DEX, este din latinul prandium.
If a word sounds close to Italian, Spanish and French, it is probably because of the latin origin, no borrowing.
If it was borrowed, then it is most likely from French (see furculision :D).
So are you supposed to say prânzul or prânz? In english I've never heard anyone say "we eat the lunch" with the definite article.