I was bored and I wanted to know how to force a friend quest, so I decided to figure out which endpoint is used to do so.
The method I used was using Proxyman to intercept the app's requests and decompiling the apk to read the .smali files.
The endpoint that the APP uses is https://android-api-cf.duolingo.com/, sometimes https://android-api-cf.duolingo.com/2017-06-30/ is
used to indicate the API version (I think?).