How do you leave a language like once you've started?
"* Go to your settings page
Go to your "Language" page by selecting Learning Language from the menu on the right of the screen.
Click Reset or remove languages under the big blue "See all language courses" button.
- Select "Reset Progress" (blue button) if you want to start the tree from the beginning. Select "Remove" (that's the big red button) if you want to remove the tree and any traces of it, such as the small flag next to your name or on your profile.
Please be aware that removing or resetting a course cannot be undone.
If you remove a language, you can always add it back later, but all your progress will be lost and you'll have to start the tree from the beginning. "
(from the Help Center article here)
A tree is the entire course that you take. For instance, I have recently finished my Ukrainian tree, or course. They are often called a "skill tree". It's called that because in the early days of Duo the course was similar to a tree: