"A banana and an orange are fruits."
Translation:Banan i pomarańcza to owoce.
That's not a perfect sentence, but it's acceptable, at least in speech. Added.
If you said that to me I would understand: „A/the banana and a/the orange. There are fruits.”
Without declining the word „owoce” you get what I've written. In correct Polish one would say: „banan i pomarańcza są owocami” or „banan i pomarańcza to (są) owoce”.