If you are truly fluent in French, this is easily demonstrated during your interview (which will presumably be in French), the quality of your French CV and your telephone conversations/emails whilst firming up your job application. If you French is adequate but not perfect, then qualifications will help your potential employer to give you the benefit of the doubt. The French government standard qualification levels are the DELF (at levels A1,A2,B1,B2) followed by the DALF (C1 C2). It was only at B2 that I could describe myself as reasonably fluent and C1 is the level needed for University entrance or for a job. If you are planning to work as a project manager, then I imagine that you will need to be able to prepare reports, presentations and papers quickly and accurately in French. As the diplomas focus on fluency in written and spoken French, they are very practical and will serve you well. Good luck.