European language level A1 A2 B1 B2 C1 C2
Would it be possible to grade the percentage of the learning with these notifications, it would be easier to use it in a curriculum, since they ask for these standards and not for a percentage.
Completing the tree and practicing all skills until you remember them will give you a solid A2. Typically your vocabulary and grammar will be almost suitable for B1, while your listening and speaking abilities will lag behind and are only at A1 in the worst case. So giving yourself an A2 on your CV is neither boasting nor understating. At least this is my personal experience from using Duolingo in combination with language courses at schools and universities.
I agree, but the interesting part of Duolingo could be to get a real certification, allowing you to skip the necessary use of the very expensive and almost dictatorial supremacy of the "Goethe Insitiut" (or other languages school known for these excesses) ? A tool for democratic learning...
The thing you are looking for is the Duolingo Test Center, which aims to provide language certificates for affordable sums, between 20$ and 50$ at the moment. Unfortunately they are only offering English certificates right now. With the normal courses, they're all kinds of methods to cheat - just having a dictionary in the next tab, for example.