You can change your language on the website by:
1) Clicking on the icon on the top right
2) Clicking on "Settings"
3) Clicking on "Study Settings"
4) Clicking on your language in "Now Learning"
The steps are shown below:
You can follow the same steps if you're using the mobile apps.
You can go back and forth as much as you like. FluentU will keep track of your progress for each language.