"귤과 오렌지는 비슷해요."
Translation:Tangerines and oranges are similar.
It is sometimes hard to know when objects need to be translated to plurals. This is just a difference in the lanuages, I guess.
Korean relies on context, so if you are talking to someone it should be obvious what they mean. (in theory lol)
Plurality isn't needed to make general statements. "A tangerine and an orange are similar" is the exact same generalization.