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.
iOS apps & Android apps
Open the app, tap on Profile at the bottom of the screen > Settings gear wheel > My languages. You can then select a new learning language from the list underneath I want to learn.