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 near the flag icon.
2. Select another course. That should bring you back to English (or your preferred interface language).
You can also tap the + icon to add a new course in the correct user interface language. Add new courses here.
P.S. You can remove unwanted courses from your account here.