I checked out you app and its pretty generic flashcard learning and video lectures which is what every language learning app is doing (zero innovation on your part). And you demand 12 or 15 eur for "advanced features". Makes me depressed if you earn any money on that.
I am 90% done developing a really good language app which is better then duolingo (which is the Marktleader) and my goal is to integrate it with actual language classes by cooperating with language schools.
The work is hard but I believe in increasing efficiency of learning by a factor of 10x - then people like you pump out these apps that dont even do anything new and demand 12 eur for it.