I'm creating an app to teach foreign language. If the user gets the question wrong, should it repeat (if so, how many times) or come back later (ie shuffle back into the deck)?
Actually this depends on the system requirements. However, why don't you just adjust the system so that allows the learner to choose if they want to shuffle or repeat. You can go further by also add an option to show the answer. But in language learning coming back to the information later on regular intervals provides continuous activation to the responsible neurons which helps with the information retention.