Make the language part open architecture so others may collaborate on it, while leaving the learning engine intact.
This way, others can develop both user interfaces and lesson plans for such important languages as Mandarin Chinese, Japanese, Korean, Vietnamese, Cambodian and other non-European languages, which are way over-represented in your product offerings.
To make it even more of an incentive to YOU at MOSAlingua, make it premium-only for the first few years, so you recoup whatever money you have to spend on making such architect changes.
If you need guidance on doing this split, Windows software development documentation includes guides to design applications so they can be made multilingual a lot easier. Your apps and programs can be designed that way as well, and then you can leave to others to contribute (and vote on) correct translations for both interface and content.
Legal disclaimer: These are my original ideas, except where otherwise mentioned. I have no Intellectual Property interest on the ideas above, except that I reserve to myself the authority to use these my own ideas elsewhere with no obligation to MOSAlingua or any other party who may have come into the knowledge of said ideas. I hereby release these ideas to the public domain.