I think in English we would rarely say that "the apples are bitter" we are more likely to say they are sour. Perhaps it is different in Romanian.
They are different tastes, though. But I agree that apples are generally sour not bitter and "acre" is the word you'd use. Maybe this sentence really deals with some rare bitter apples.
I'm wondering. I wrote "The apples are bitter." instead of "Are the apples bitter?" but the first scentence has been accepted too...