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.
Sam's suggestions are good and I think testing what constraints or premium experiences you can tease in the product experience itself can lead to some increases in conversions.
My advice would be to A/B test the copy on your premium page. I'm not a user of your app (but I understand what it does) and the way you describe the premium benefits just don't feel too compelling. Some small copy changes could result in increases in conversions.
Some social proof (pictures of customers with testimonials or even just testimonials) about how much better the service is with premium could really help.
Also, I hope you're already using analytics to determine how many people click on the "Get Premium" button but who don't complete the purchase. My sense is that the multi-point purchase plan might be leading to fewer purchases. You might want to experiment further with fewer plans or a fixed plan.
I have been building and selling digital products online for almost 3 years. I have tested and re-tested many models of pricing and found what works and what doesn't.
Here's my thoughts on your issue:
I would say that you need to determine what your goals are. Are you trying to get the most users, or are you trying to get paid users?
I took a quick look at your page and I had a hard time even finding what or how to upgrade my account, and when I did find the page, I was not given any incentive to upgrade at all.
I would remove all the places where you say 'It's Free!', because although you do have a 'lite' version with no-cost to try, the product itself is a paid product. This not only will enhance the value of your paid product, but will also greatly increase the value of what your are giving away for free with your 'lite' or trial account.
I feel that you need to define who you are selling to and what it is they are getting in return and then you need to ensure that there is clearly stated and obvious value to upgrading.
I am more than willing to jump on a call with you, should you have any further questions or if you'd like to get together and build a more effective sales funnel.