* The market for actually useful, non-gamified learning apps is smaller than, say, Duolingo.
* Yet the market for bullshit apps is too saturated. There are maybe 50 such apps for each major language already in the App Stores.
* As a customer I'd be happy to pay for serious, boring learning apps, and I believe such serious customers exist. (but in much smaller numbers)
* Market for serious, boring language learning apps is underserved. (for German there are apps like Readle, Vocabeo, Vocabuo (yes, lol naming), DerDieDas that cover specific niches, and (afaik) only DW has a quite comprehensive actual learning program)
I believe potential customers like me exist, but our numbers are much less than "learn Spanish in 5 minute games" crowd and our expectations are higher too. Up to you to decide if this is a valuable niche to serve.