"Do you like to eat Korean food or Japanese food?"
Horses prefer Western food.
Okay I'm guessing you mean 吗 (which I think is always a neutral tone). It seems to be specifically for yes/no questions.
还是 is an or used with questions so by using it you are implying your statement is a question.
Because you already have one interrogative word here, which is 还是。 The same pays in this sentence 他是谁? (Who is he?)。When you have here 谁, you don't need 吗 (not 马 as you wrote (this character means horse, you must pay more attention).
No unless it is a question for recalling the preference of the listener that he/she has told you before.
Note: There is also use of 还是 in an affirmative sentence. Seeing 还是 does not always mean that it is a question. 还是 in an affirmative sentence means rather, still, better still, depending on the context.
Let's have Korean food... urh, no, Japanese food rather.