"I'm not interested in the French course."
Translation:No me interesa el curso de francés.
No, this version doesn't need an "en." This sentence is literally "the course of French does not interest me." "Interesar" is used like "gustar," which, as you probably know, has a word order different from English.
You could possibly say, "no estoy interesado en el curso de francés," which would use your "en."
See the explanation from marcy65brown about how the verb interesar is used in Spanish.
In your sentence, you used the 3rd person conjugation 'interesa', so your sentence translates to something like 'I don't it interests', which has 2 subjects (I and it) but no object.
To use the verb interesar in Spanish, you need to say something that would translate into English as 'It doesn't interest me'. So the 'I' subject pronoun in the English translation becomes the object pronoun 'me' in the Spanish version. A mi, no me interesa. Or No me interesa
Yes, it's wrong. The subject of the sentence is "el curso de frances." It is saying "The french course is not interesting to me". It is incorrect to say, "In the french course is not interesting to me". Used this way, 'interesar' is just like 'gustar'.
The following is correct: "a mi no me gusta el curso de frances" it is incorrect to say: "a mi no me gusta en el curso de frances"