Most of the time I avoid to stay in Paris too long. I just can't stand this city. Noisy, expensive. Some people like it. I don't. If you can, spend more times in Nice. Better weather, better quality of life, not too far from the mountains, italy, nice places to visit such as the Oceanographic Museum in Monaco. October is a mixed period with the weather but diving should not suffer from that. My only dive in France last year was by the end of october. It was a night dive. The air was cold at night but the water was warm and very clear (you could see the moon by looking at the surface from 150ft !).
For your french, don't worry too much. Many people in France know english (Nice is heavily frequented by english and american people as well as italians and russians) but obviously it's a personal reward for you if you can handle a conversation in french