What language level can I achieve using Duo?
If I learn all words and structures tought in the Japanese tree, what JLPT will I be?
When I finish the Japanese tree (and learning everything), does it worth to use LingoDeer app? If it doesn't, is there a similar app that I can use?
[Sorry about my English, feel free to correct me :D]
The Japanese course on Duolingo was specifically designed to teach up to an N5 level of Japanese fluency. As for further studies, I have not used the LingoDeer app, but I have found these online MIT courses to be helpful (you will need to purchase textbooks separately): https://ocw.mit.edu/courses/find-by-topic/#cat=humanitiessubcat=languagespec=japanese.