How is it possible that you provide 'dish' as one translation of 'prato' and, then, mark it as incorrect when I use it? This is thoroughly in consistent and, in any case, what is the essential difference between a dish and a plate?
As for duolingo, it's probably an inconsistency in the dictionaries used for each language. As for your language question, "dish" does dometimes mean something different than "plate." A "plate" is pretty much always an object upon which food is served. A "dish" can be an object upon which food is served or the food itself. Here's an example: "My favorite dish is spaghetti." You couldn't say, "My favorite plate is spaghetti." But you could say both, "I serve spaghetti on a dish," and "I serve spaghetti on a plate."
"Dish" and "plate" can also be verbs ("to dish out" or "to plate food") but that's another story.
It is not Duolingo that is inconsistent but rather English. In Portuguese they have the prato do dia which in English is the dish of the day. Still served on a plate. But the word dish in English comes the Greek/Latin, discus/diskos and probably came to English via the German tribes while plate probably came from the Normans aka French (though originally Greek too, platus... plat – originally for armor not so much eating).
So for the English purposes, sometime prato in Portuguese means plate and sometimes it means, dish and this is part of contending with a new language is accepting these differences... cause there will be many others! :D
It's a DISH!!! That's how a Brazilian would FIRST translate! We use "prato" meaning dish much more than plate. Ex: What's your favorite dish? -> Qual seu prato favorito?
The OBJECT PRATO is PLATE. The favorite DISH is also called Prato.
But dishes are broader. You can wash the dishes, and that would be washing more than just plates. In that case DISHES is LOUÇA.