I cannot get back to my main courses.

Go here to see a list of methods to switch among courses (which includes the method described in the Help Center section "My account is in a language I do not understand! How do I change it back?").

How to delete?

If you didn't add any other course from French, then you can't delete this one.
Otherwise, see Help Center section "How do I delete a language from my profile?".

• Hover over "wjbet"t on your dashboard
• select "Réglages" (2nd in the dropdown menu that appeared)
• left-click "Langue actuelle" (2nd down on right)
• in the "Langue d'apprentissage" dropdown menu on the left, select any of the "I want to learn . . . " choices
• left-click "Sauvegarder les modifications" (green button towards the top)

[added] Sorry, I told you how to get back to English but not how to delete. Slight omission, there. :( jrikhal has it covered. Have you been successful, wjbett?