My experience as an estate agent in France suggests that an ‘English-language’ version of a mandat de vente is purely informative. It has no legal value and only the French version (which should be signed concurrently or later) will be considered as valid.
In this particular case it would appear that the agent (or probably the agent commercial) is acting in bad faith and is trying to make you sign up to a forfait rather than a set percentage of the eventual sales price. If you are not sure just what status this person is working under then just ask to see their carte professionnelle d’agent immobilier. If they don’t have one then ask to meet with the agent immobilier who is sponsoring them. They should sort it out quickly.
I went into this business as I was so incensed with the way French agencies treat their clients: very little information, poor communication, outrageous rates of commission, etc etc. No agency should charge more than 4%. Very few vendors or buyers realise that they can negotiate the rate of commission downwards if they feel it is too high. What leverage do you have? As many people on this thread have pointed out you can just walk away from the offending agent or agency. There are plenty more out there.
You are the vendor: it’s your call whether to sign up with these people or not. And you can take their mandate and, before signing it, just strike through the offending part of the phrase:
“Vendeur un forfait de la somme de xx,xxx euros ou 5%percent du prix négocié” and just leave “5% du prix négocié”. (and make sure you initial the margin opposite this strikeout). If they want/need your business they will accept this.
But in the end, if you have problems with an agency BEFORE you even start to sell the property … it’s probably better to cut them loose immediately and find someone more motivated.