Having worked overseas for UK tour operators for most of my life, being on emergency call 24 hours a day, 7 days a week is an entirely normal obligation, so the fact that you are only obliged to have it on 6 days is a bonus! It is ultimately why I left the industry - when I returned to the UK for a funeral but was told I still had to be on call. To top it of a client called just as we were about to go into the church insisting I book him on an excursion the next day, even after I explained where I was.
Whether or not it is legal, I have no idea but it is industry standard. I don't know what you actually do but it is rare you will actually work 35 hours and most employers will argue that this is why you are on call, it is the nature of the beast. Imagine yourself on holiday, you or a friend are injured, you don't speak the language, you don't know what to do but, hey, you can't contact the rep as it's her day off. As a rep you normally are free for several hours each day, you have no one looking over your shoulder, its called swings and roundabouts, it is up to you whether you want this kind of lifestyle or not. On the other hand, if you are working in the company office doing accounts all day, or cleaning tents in the blazing heat for all of your 35 hours, then it is unreasonable to expect you to also be on call. It also depends on your level of seniority and pay
Overall you are fortunate to have a French contract and it will stand you in good stead, don't expect to take the company to court over hours and get another job in the industry - harsh but true as they are all the same.