To get users, we started a meetup group to teach people Korean. https://www.meetup.com/San-Jose-Korean-Language-and-Culture-...
We stopped working on the app, but continued with the meetup group (3 years now and almost 1000 members).
Our conclusion is that the most effective way to learn a new language is a simple commitment to showing up regularly to language events. Meetup.com or your local public library will usually have regular (and free) events. Once you have that commitment, picking a great app will help expedite your learning.
Excited to see an open source language learning app!