Ideally, users should be cautious in choosing the projects to which their dedicate their interest and enthusiasm (choose those which belong to the community)... Under the assumption that the game cannot be made open source (or that it is impractical to give a share of the game's profit to the contributing users), I imagine that a good solution would be to republish all translations separately under an open license (in case they can be reused, very theoretically, for something else... maybe as an aligned corpus to train a machine translation system), and make sure that the users who contributed are prominently credited.