Yes, it does not cover every language but the intent is to address that ( perceived ) complaint.
I disagree that an actual coursebook is more helpful.
I used Duo for 6 months prior to moving to a Germany. It gave me an incredible confidence boost to be able to at least understand what was going on around me.
After living in Berlin for 6 months I did a course and I was far ahead of others who had taken formal courses before.
It has been 2 years since I left Germany, I still do my Duo daily and have continued doing German courses.
It has helped maintain my minimal ability.
> it also likely depends on person
Indeed. My partner at the time did the same as me and Duo did not work for her at all.
GitHub repo says it was archived 2 years ago: https://github.com/Netflix/titus