You can learn multiple languages at the same time and save your progress. If you accidentally changed your interface language to something you cannot understand, please check out the article My account is in a language I do not understand! How do I change it back?
If you are simply looking to add or switch to a different Duolingo course, here's how to do it:
On the web
On the iOS and Android app
Tap the Flag icon on the top-left to change your language course.
Simply select the course or language you would like to switch to. Note that if you change the base language, the app will change to that new language. For example, if you were learning French for English speakers and then decide to switch to the French for Spanish speakers course, the app's interface will change the base language (Spanish).
Note: You will never lose progress by switching around to another language. You can go back and forth as you please, as long as you are in the same Duolingo account. If you appear to have lost progress, please make sure that your Duolingo is signed in to the same email address/account you had before.