There are several large white Livestock Guardian Dogs that look similar. The Great Pyrenees is the first we think of, and like somebody said, they are only interested in threats to their wards, usually sheep. My friend breeds and raises these on her sheep farm and they should be friendly towards people passing by. There are a few more primitive breeds that I heard can be more aggressive when not raised properly, like the Akbash and Kuvasz. But since you are in France, my first guess at breed would also be a Great Pyrenees. Of course, any dog can be aggressive when not properly socialized.
I have herding dogs. The pointy eared shepherd-types. With my type of dog the best thing is to stop immediately. They just want to catch whatever is moving fast, it's prey drive for them. My Belgian Malinois bit at the tire of a bike when she caught up with it and gave it a flat. (She was a few months old and only had the one chance.)
http://clgoetz.8m.com/boaz/boaz/
These big white dogs shouldn't have a prey drive. But since they bit hikers also, it sounds like there is nothing you could have done in your situation. I hope the police can do something about them. Laws in Europe seem to be a lot more strict about these things then here in the States.
http://www.lgd.org/