Is what I did considered cheating and a sin, when my French language instructor informed me —through my colleague— of the two exam topics that would appear, and I studied them and got a good grade because of them?

If a teacher specifies exam topics in a way that violates exam regulations, he has betrayed trust and committed an injustice that corrupts the student evaluation process. However, if a student benefits from this action without seeking or requesting it, it is not considered cheating and there is no sin in it, as Sheikh Ibn Uthaymeen issued a fatwa (definition) permitting a student to benefit from an answer he hears unintentionally. It is advised to focus on current studies and strive diligently in them.