10 years ago when I moved to Germany I had a similar learning system. I read books, I highlited the words I wanted to learn and once or twice a week I wrote them up in a small book. After a while a wrote a small VBA flashcard app for tracking my progress and randomizing the words. And that took a hell lot of time. With an app like yours I could have saved 30-60 minutes every week (2-4 hours every month) which is worth of 20-40€ a month (on minimum wage). That´s why a recommend you to spend way much more time marketing your app and finding the right customers, which means:
- already learning languages (don´t get fooled by "If I had enough time, I would use it")
- learning languages because they have to (living abroad or working international)
- saving 2-4 (or more) hours a month is worth them at least 10€ (no students spending more money on boose than on books)
IMHO you can find a lot of young professionals on forums like the "new in wahetever city" groups or expat groups in facebook.
Good Luck!