Last updated: October 2, 2017
If your Duolingo account is displayed in a foreign language and you need to switch it back to English (or another language), don't panic! The UI language can be changed.
1. Mouse over the dropdown next to your avatar.
2. Select another course. That should back bring you back to English (or your preferred interface language).
P.s. You can remove unwanted courses from your account here.