I think you could market it this way :
You are in the language learning industry. In learning, pratice is key. Learning is a journey (takes a some/lot of time). (expertise is 10 000 hours of work/experience)
What is good about learning is there are levels (ie. beginner, intermediate, advanced). For the software/app maker, it means there are different needs to answer. For your potential user, it may mean, there is a potential long term gratification "I am a beginner user today, and tommorrow I can become an advanced user". "I can make progress."
- How to make your visitors sin up ? Break down the different japanese apprenticeship levels. Show your user, examples of instant gratification. Even if they are beginner, they can get something.
- How to make your users stay and use your app ? Gratification at every little step they make. ie. Every time they write a good answer.
In other words, I think adding a layer of gamification would be interesting for you. It would spark your visitors interest, grow your user's engagement.
- How to make people come ? Maybe by allowing your users to challenge their friends. Ervery time your user reach a milestone, allow him to post on Twitter/Facebook something like this : "In two weeks, I allready completed X,Y,Z levels of Japanese. What about you ?" (far from perfect, but you get the point).
Having a freemium business model is a good idea. It lowers the barrier entry for the user.
In addition to that: - Define the the user tipping point : when are they addicted. What does it means to be addicted to your app. - Define the right metrics to analyze.
Many startups in the learning industry have successfully implemented a gamification layer in their application. You may take a look at codecademy.com and veri.com
HTH and Good luck.
PS: Great ratings show you're into something.