"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."
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"
I wrote "No me interesa al curso de francés" because previous examples used the 'a'. I am completely not clear on why this is. The other example was "We are interested in your question, Juan" which did require the 'a' even though the object they were interested in was not Juan, but Juan's question.
Can anyone please clarify the use of 'a', and when it's needed? I'm getting a bit Pavlovian, stabbing at the answers and hoping for the best without a clear understanding. It seems like a 50-50 coin toss, heads I'll add an 'a', tails I'll omit it and see what happens.
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