Is there any sort of auditing or quality control for subtitle files added to your database? If you are able to review, edit, and 'certify' some premium subtitle files that might be a reason to go premium. You could offer one free premium subtitle download pack for every free user.

What do you have in place for usage tracking in your app? Do you find that users download the same subtitle files in multiple languages? If you have some usage tracking in place it'll be easier to see what some trends might be.