You might want to consider lifetime pricing too. The prospect of having to dig out a credit card and make an online payment every month or year is annoying to many, and they'll be much more willing to pay if it's a one-off thing. As mechanical_fish points out, language students often don't study the language for too long - a year or two - so it may even turn out beneficial for you too. Of course this may not work for your specific application, but it's worth a thought.
On a separate note, your app sounds really interesting, especially since I'm learning Japanese too (currently using Anki for vocabulary learning). Also I've been thinking recently about how to apply SuperMemo-style learning to more complex language learning than vocabulary, such as grammatical structures, and it sounds like you're working towards that sort of thing. I'd like to take a look at your app if you're opening it up for more general testing. If you're interested in discussing further, please email me: callosum at gmail etc.
On a separate note, your app sounds really interesting, especially since I'm learning Japanese too (currently using Anki for vocabulary learning). Also I've been thinking recently about how to apply SuperMemo-style learning to more complex language learning than vocabulary, such as grammatical structures, and it sounds like you're working towards that sort of thing. I'd like to take a look at your app if you're opening it up for more general testing. If you're interested in discussing further, please email me: callosum at gmail etc.