I finished my trees in English from German, In French, Italian and Spanish from English an in French from Spanish.
One Problem which is obvious, the profile never shows all options if you are working on different mother tongues. The second is the profiles shows very often too low xp values.
But the main problem: I started with Italian from Spanish some weeks ago. Within short a fluency of 8 was reached. Than it dropped rapidly to four although I do on average more than 100 xp in that language per day. Yesterday I spent some 60 points on repeating and the fluency climbed to five. After a further 300 xp on repeating, leaving nearly everything golden, I was back on 4 again.
To be honest, I like the gamification. So i hate mistakes in the system making it an ever-loosing game. So in the end this bloody fluency counter which is making me angry will persuade me to leave Duolingo in the end.
obviouly the system does not like working on the same language from different mother tongues. Even if the system would only show the values of the actual mother tongue used, it could have the results separate and non-interfering.