My last (for the session) French conversation class met today. It has taken a number of weeks but I now understand the accent of my teacher almost completely. As with English, there are so many variations that one just has to be patient with one's ears until they catch up with the brain. Since I was originally exposed to French in Lyon I can identify with both northern and southern sounds. But I can also become very confused (especially the "r" sounds).
I don't know if this works for everyone, but when I hear someone speaking English with an accent often I can't understand them until I know what their native language is. My brain somehow figures out the pattern of speech sooner when I know its origin. So, sometimes I don't understand someone in English until I find out they are German-speaking and somehow that knowledge makes comprehension easier for me (especially since I studied German for a long time).
With French, I have apparently spent enough time in the country and with French people to get confused with those regional accents. But if I know someone is from the south of France, voila! suddenly their speech is clearer to me. Brains are amazing.