Are you actually going to be living in France while you do the work - self employment in France can be offputting due to social charges. If you do live in France, for freelancing or contracts , it can be much less risky and less stressful to work through a portage (i work through
Challenge & Co Portage - an alternative to self employment in France in English !) which basically allows you to act like a freelancer, but pay any social charges PAYE (ie there is no risk and you dont have to register as self employed). If the head office of the company you are doing the work for is in France, even if it's a small co, you will need to charge them the French equiv' of VAT if you live in France.