Years ago, I went to live and work in Strasbourg. My French was… rudimentary, school-level, but after a few weeks I was picking up the rhythm and following along. Then the grand chef came up from Paris. During the night out entertaining him, I asked him to slow down a bit as I was struggling with his accent. He completely lost it, insulting the locals as peasants, and claiming the accent was theirs not his. Kind of put a damper on the evening.
Obviously the Marseille “dialect” is recognisable, but otherwise, travelling throughout France, and even the French-speaking parts of Switzerland, I could understand folk.
Actually, you don't. Strong regional accents are pretty rare compared to the UK or Germany