'Prato do dia' implies 'plate of the day', which in Portuguese restaurant terms refers to 'Today's special'. The Portuguese restaurants here in South Africa are very fond of cooking their traditional dishes on a large scale to serve to their customers. Carne assado - roast meat. Galinha com piri-piri. A butterflied chicken grilled over glowing coals after being marinated in lemon, garlic, olive oil, bay leaves and hot sauce. The best cuisine in the world!
There is also Dish of the Day which goes across many languages.
In Portugal most restaurants have a Prato do Dia at a set price which is their main dish for that day often including a drink, dessert, and coffee, along with several daily set menu items that can be ordered separately or instead.
But for sure, "main dish" or "course" is common in English, with the accompanying "side dishes" which on menus are often referred to as, "sides" (think fries, toast, side salad, olives, but also just anything additional like an extra egg for breakfast, or more butter), but in general, at home the main dish is usually the meat (or meat analogue) or perhaps a casserole, savory pie (think calzone or pasty here), with perhaps some vegetables, and salad served, "on the side" along with it. Note that a main course in a restaurant and a main dish in a home are treated a bit differently:
The use of the word "dish" to mean a food has its origin in the container that a food is served in, which is to say, a physical dish. This word ultimately comes from the Classical Greek "diskos" meaning "disk or platter", which passed into Latin as "discus" with the same meanings. Its use in English to mean a type of food started happening around 1450.