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 in your settings.
1. Go to your settings page.
2. From the menu in the middle, click the arrow to show all available courses.
3. Select your preferred language.
4. Remember to save in order to make the change. Click on the green button on the top (the blue one will take you to the courses page).
There you go! That should switch your language back to English (or other preferred interface language).