Arika Okrent highlights a map “made in 1847, before French had truly become the language of the whole of France. The oïl languages are outlined in pink, the oc languages in blue. The rust brown in the northeast is Celtic, the green, Germanic, and the yellow, Basque”; you can explore it in zoomable form at the David Rumsey Map Collection. It makes a fine companion to Graham Robb’s The Discovery of France (see this post). Thanks for the link, Martin (and marie-lucie for the book)!