To a supervisor? Because I'm talking about mail sent to a supervisor. You know you have a right to write to a supervisor in French only.
I don't know your department very well. However, I do know another department where an individual I know as well stopped writing to her supervisors in French because people "looked at her strange". The text was sent for translation and came back translated but didn't exactly express the meaning she had wanted to convey. Do you have that difficulty?