Immersion works much better if you have the bases because you can't have a conversation if you don't have the minimum level and nobody want to talk and waste their time to someone really bad (ain't nobody got time for that), especially in France. At the end of the immersion, worst case scenario you will have a better listening. Personally when I went traveling in South America I improved a lot, people don't talk English so you have to make a huge effort and that's how you learn, but I had learned the bases at school and read the grammar few weeks before going.