How did I get Japanese already?
Why did I get Japanese already, when it's not released yet? I wasn't able to complete any lessons and realized it's not on yet. What a mystery. All the other trees are legitimate, but I've been away from them for a while and they all went blank. Is this the eventual fate no matter how golden some trees are? When I realized this, I stopped pretending to be so perfect all the time.
Japanese has only been released on mobile. You can't access it on web yet.
Yes, skills decay even after you finish the tree. After all, simply completing the course won't stop you from forgetting things.
Get the ARC Welder Chrome extension, https://www.addictivetips.com/ubuntu-linux-tips/run-android-apps-on-linux/ and then download the DuoLingo APK, https://www.apkmirror.com/apk/duolingo/duolingo-duolingo/duolingo-duolingo-3-57-0-release/duolingo-learn-languages-free-3-57-0-android-apk-download/download/ This allows you to run the DuoLingo Android app in Chrome. I just tried it on Ubuntu 17.04 and did a few Japanese units.