The verb "to lunch" exists and therefore it should be accepted, even if it sounds a bit awkward in this sentence. https://en.oxforddictionaries.com/definition/lunch
Correct! In italiano NON è corretto dire/to say IO MANGIO PRANZO But only IO PRANZO or IO CENO. Anyway you can write only IO MANGIO but without specificy what is it, lunch or dinner, except breakfast. I mean breakfast because in italiano I write Io FACCIO/MADE COLAZIONE. I'm sorry for my english but i'm native italian. I use Duolingo in two versions!
To those struggling with this, it is VERY unnatural for English speakers to have to infer words that are not there. Stay with it and practice. If the sentence is truly "The cook eats lunch," then we expect to see "mangia" in there. If it's "The cook has lunch," then we want "ha" in there. It is a very very long process to learn to do without these words. Non-English languages have this all the time. The very first sentence I saw in Latin 101 was "Villa est villa romana." So I said aloud "House is house Roman." 4 words, and I knew the meanings of them all, so I said them all, in order. The teacher thought I was trying to be funny, but I honestly did not comprehend that it means "THE house is A Roman house." And the adjective coming AFTER the noun? Extremely difficult to grasp in English, but so natural for other languages.
if pranzo is the main meal of the day then translating it as 'lunch' seems odd. Lunch is never the main meal of the day.
The main meal of the day is dinner, if it's in the afternoon then the evening meal is supper. If the main meal of the day is in the evening then the midday meal is lunch.
Because Italian has more than one word for it, just as English has "dine" or "lunch" (as in "ladies who lunch"). It won't hurt you to learn "pranzare," which is used in Italian far more often than "to lunch" is in English. You're trying to learn vocabulary, right?