That's second hand information from your friend and not reliable. Ask your French teacher about what you heard. They are humans, after all.
My French Teacher Badmouthed Me In Spanish With One Of The Girls Who Just So Happens To Be In My a Sixth Period French Class. He Said I Was Really Shy And How That's Stupid And I Should Really Speak Up. Should I Tell Guidance What He Said?
If your authority were equal to that of your French teacher, I would say confront her and ask him if it were true. (You would be over 18 at least.)
You are actually old enough to stand up to adults---whether they are right or wrong---but you may not feel that power yet.
Assuming you are not 18 yet, I would suggest talking to your counselor. You can simply say what you heard and ask for the counselor's help in deciding how to solve the situation.