Mirlibak,
I met people in France who couldn't speak French, but they usually found others willing to help them make reservations. One woman from South Africa had her mobile phone and said she easily found other pilgrims who would use it for her to make onward reservations. Janet, from this forum, used to go into the Tourist Offices which are nearly always very helpful. And sometimes gite owners will help by ringing ahead. I could write down a few sentences in French, but you might find it hard to pronounce the words if you are not familiar with the language.
It isn't possible to sleep on the floor in France - once a gite is fill, it is full. However, there always seem to be people who will help in this situation in France. The tourist office would be a first place to ask, but also maybe the mayor's office.
You will need to carry cash: hardly any of the gites will take credit cards. There are many towns along the way with ATMs, though be sure to think ahead a few days if you are about to walk a more rural section of the trail.
All the best for your walk!
Margaret