Yep, you hit the nail on the head. It's likely that this product will always have a highish churn rate because of that.
But with low running costs, and a large enough supply of new users, it could still be a good business. Also, there are hard-core users who really do use it all the time, for whom the churn rate will be much lower.
I'm a language learner myself, and I also frequent various reddit language subreddits and read all the stories of other people.
My belief is that "don't have time" usually means "I've lost motivation after the initial high". A huge problem with language learning is that unless you have moved to another country or otherwise in an immersive environment then it becomes quite difficult to keep motivation and discipline high. The reward of "I can read/write/speak/listen a little better than last month" starts to pale and people realize they actually don't use their new skill on a regular basis. And like many things, getting to the next proficiency level takes increasing amounts of work. The flip side being that for X weeks of study less and less progress seems to be made.
What I'm getting at is that this churn is not a problem with your product, but a broad problem with language learning. Anything you can do to solve (or lessen) these problem is a big, uncertain task. But it's probably the only way to reduce churn. The upside is that if you do manage to make headway then you've done something major. Really major.
You might want to look at structuring your pricing in a similar manner to gym memberships given that the product has a similar usage pattern. There was an interesting Planet Money[1] podcast recently that noted that many gym memberships are structured to be yearly contracts ... because people like that idea of putting money down so that they feel committed. You might be able to tap into some of the same psychology.
Another approach I saw a gym take recently was having a relatively high price but then you "earned" back a portion of the fee every month by meeting a certain activity goal.
It seems like it might be complicated to administer, but I think the psychology works well for an aspirational product because people will like the idea of making a commitment to a personal improvement but will probably over-estimate their future activity.
You might also want to look at the techniques which gyms use to extract maximum revenue from aspirational users. In particular, heavily encouraging/discounting annual memberships.
(Then again, extracting a year of revenue from someone who only uses it a month or two might not be your cup of tea.)
But with low running costs, and a large enough supply of new users, it could still be a good business. Also, there are hard-core users who really do use it all the time, for whom the churn rate will be much lower.