I agree, it sounded wierd to me at first. But, i thought it out and to say "I eat a pancake with banana" doesn't sound weird anymore. You're not necessarily implying any specific amount of banana, just that the pancake is being eaten with it. I think perhaps if the Romanian sentence had said "un banana" then it would be more specific. But, it only said "cu" which again, only saying that it is with banana, not how many. I'm certainly no linguist, so just my two cents. Hope it's useful. Hai noroc!
Yes, it's wrong. The meaning of "I eat a crepe with banana." is "I eat a crepe filled (or made) with banana puree or banana slices." The Romanian sentence "Eu mănânc o clătită cu banană." has the same meaning, even if you use the plural form of the noun (banane).
If you say "I eat a crepe with a banana." then the Romanian sentence will be "Eu mănânc o clătită cu o banană."