Tip: Quickly switch courses/trees using the Windows app
I'll show you how to set up a one-tap shortcut for switching base languages on Duolingo, using the Windows app as the trigger.
For anyone who likes to work on more than one course ('tree') at a time, switching back and forth can be a little annoying. Especially when you choose to do a 'reverse' tree, i.e. setting the 'native language' to your target language, and 'learning' your native language (or another!). Some people do this for an extra challenge because of the inbuilt bias Duolingo has against translating into your target language. Switching between EN>DE and EN>FR for example is not too hard, but to switch to DE>EN you have to click through a few more options.
However, if you use Windows (version 10 on your computer, or version 7+ on your phone) you can get the Duolingo app for Windows and use a helpful feature to quickly switch between trees: Pinning course tiles to your Start screen. As far as I know, the Android and iOS apps do not have this feature.
When you open the app, you can side-swipe between trees that you're learning in the same base language, i.e. EN>FR and EN>DE, but not the reverse tree. In fact, there's no way to change to the reverse tree from within the app itself! However, here's the workaround, which actually ends up being faster than using the web version to switch once you've set it up:
Open the Duolingo app.
Navigate to a course you use, and click the 'Pin' command (this may be inside the menu at the lower-right of the screen). This creates a shortcut on your Start screen directly to that course (e.g. EN>DE).
On the web version, go through the normal process of reversing your tree (changing the base language of Duolingo).
Open the Duolingo app from the main icon (not the new shortcut). It will now also be in the new language.
Create a new pinned shortcut for the reverse tree (e.g. DE>EN).
That's it! Now, whenever you want to use Duolingo in a different language, just launch the app using your preferred shortcut. This will register the change to your account, so even if all you do is launch the app but not do any lessons, the next time you open the web interface it will also have reversed the tree. So, you can just use the app shortcuts as 'switchers' for the web version if you want (this is often the best approach anyway, as the app currently lacks many helpful features).
TL;DR: Pin shortcuts to your Start screen to toggle between languages. Clicking the shortcuts will change the language even on the web version.