When you've logged in to Babbel, your current learning language is indicated by the flag at top right of your Babbel homepage. Click on that flag and a drop-down menu will appear with the option to either change or add a new learning language.
When a language has already been added, it will also appear in the languages list under You're learning on your Babbel homepage. You'll see the option to Switch there, if ever you want to change to one of those languages.
In the apps
If you want to try a different language, simply download the relevant app. Babbel's are completely free to download via www.babbel.com/mobile