"Een sinaasappel is fruit."
Translation:An orange is fruit.
"fruit" in Dutch always means the class of food (fruit as opposed to vegetables or meat etc) and is not countable. A specific fruit (as part of a plant) would be "vrucht".
Is the fact that "fruit" in dutch is not countable the reason you dont say in dutch "....is EEN fruit"? In English "an orange is A fruit" sounds better to me.