"The plate of the day is fish."
Translation:Il piatto del giorno è pesce.
Well, the whole sentence is in the singular so it makes sense. Keep also in mind that "pesce" is a collective noun in Italian too, although with different usage rules than English.
Compare to Spanish... We say "El plato del día es pescado", not "El plato del día es pescados"
Does this have the meaning of: Today's meal is fish? I have never heard this expression before of "today's plate" nor in Italian, nor in English. Thanks for your answer in advance! :)
No, "il piatto del giorno" usually refers to a restaurant's daily special; it's often a dish not on the menu which is only available on the specific day, or that is better eaten on that day (for instance, the restaurant just restocked on fresh fish). Wordreference seems to translate with the French loan "plat du jour".