Completed simultaneously Spanish and German skill trees!
Spanish was a totally new language for me; I finished the skill tree without interrupting on 17th level (140-days streak, 10686 skill points, 1571 words)
I did not have to learn German, so I did the tree just to test the system and myself, achieved 11th level (only 3829 skill points, 1710 words).
I am still just a beginner in Spanish and have a native level in German - so DL learning level shows little about the actual knowledge. Completing only a DL skill tree without immersion is a good basis for further learning. There remains a lot of work to achieve e.g. intermediate level.