"One apple, please."
Translation:Bitte einen Apfel.
It is never Enie, which is for femine words. Apple is masucline. Ein Apple (an Apple) Ich esse einen Apfel ( I am eating an Apple). Einen is used because the apple is being acted upon. It is being eating. You are (using the accusative case) 'accusing' someone of doing something to the object = the apple
Because the sentence is in the accusative case. The subject is implied. You are asking for an apple, what would mean that you are saying something like "I would like an apple, please". Masculine subjects, such as apple, takes the Einen instead of Ein. If you said "Einen Apfel, bitte." that should be correct
"ein apfel, bitte" was accepted as a response for me.
"One apple, please." does not have a ? so to me it implies a command to someone or response to a question with a polite please i.e. store clerk asks you how many apples do you want. Would a person never respond like this in German?