Assuming you want an immersion experience rather than a course, find a small, rural, no touristy town. According to a recent EU poll, only 36% of people in France speak basic conversational English, and they are concentrated in big cities and touristy areas, so that doesn't leave a lot of English speakers in the small towns.
I found this out in Germany (I wanted a German immersion experience) in 2000, when I went to a small town in the Oberallgäu. I was disappointed when my host spoke English (he had been a chef on an American cruise ship and his wife was English), but few other people in the town spoke English. The high point was when I was out in town at lunch time and sat alone at a 6 person table at an outdoor restaurant. A non-English-speaking German couple sat down next to me, and we had a stimulating (for me) conversation in German. Up to that time, I had been anxious to speak German, but it really gave me confidence.