A cover letter (la lettre de motivation) should be crafted in perfect French, and only a native translator can guarantee you an excellent translation!
You need to write a cover letter and a French resume if, for example, you want to enroll in a university in France or find a job in a French company.
In addition, you will need an official diploma in French, such as DELF, to validate your proficiency for universities. I am also an online tutor for DELF preparation!