Best places in France to go to practise French?

I'm 18 years old and before going to uni I want to spend around 3/4 weeks in France to continue practising my French. I'm not a native speaker but am around level C1 and have been to Paris and St. Malo, Brittany but often if and when people that I speak to think that I am an English speaker or overhear my family speaking English they will speak in English to me, which is annoying because I am trying to practise my French so i wondered where in France (preferably in the south) are there typically 'less' English speakers ?