I was a linguist in a former life, so these apps always intrigue me. I've made a few attempts myself over the years. Yours looks slick and would've been an interesting one to check out if I were learning a language.

I signed-in and took a look around getting a few "Error rendering home" errors occasionally, fyi.

I was a little surprised at the CC#/subscribe page, since the prices didn't seem to be matching up with the marketing pricing page.

You might want to consider having like a sandbox account with some sample materials so people can feel the power of the app rather than depending on someone subscribing based on the video only.

Cool idea!