Issues using the Android app while in the Dutch from English beta
I had left the Duolingo web interface switched to the Dutch from English beta tree, when I went to use my phone. It complained about the current course being unsupported, which is understandable. However, it then preceded to switch me to the English from Dutch course as some kind of fallback and appeared to remove my French, Spanish, and Portuguese from English trees and switched the interface language to Dutch.
While I panicked initially, I guessed that it was worth trying to re-add one of my trees to see the effect, my suspicion being that either the mobile app was attempting to fix the 'issue' using a heuristic to guess the tree actually intended. The Portuguese course was my most expendable one as I had my lowest level, so I added that back in, and it somehow caused the issue to fix itself.
I'm a software developer myself, so I an understand how a bug like this could've crept in, and it didn't cause any data loss, so no harm, no foul. However, if a less technically inclined user were to have this happen to them, they might not have been able to correct the issue themselves.
My suggested workaround for unsupported courses in the mobile apps would be for the mobile app to flag unsupported courses as simply unavailable rather than attempting to guess what course it ought to be displaying.
Same thing happened to me, I was really not happy at the thought of losing my current progress but as you did I sorted it out in the end.
The annoying thing is obviously after it changed everything into Dutch I had no idea what the buttons said so had to get on google translate as I was worried about deleting something.
Anyway got there in the end.