For those of you who are curious, technically the oldest mosque in France is this one, in Mayotte. It was built in the 16th century, and it's unique in that it is built with corrals. Islam in Mayotte is also very relaxed and trouble-free. It's heavily influenced by India and Persia. We have 0 problems with this Islam (in fact the French Republic even officially recognizes the Islamic traditional judges of Mayotte, called qadis, which shows that French secularism can be pretty flexible, contrary to what outsiders often imagine). They've had to abandon shariah law when they became fully integrated with France in 2014 however.
And these are the qadis and their Muslim courts in Mayotte. Since shariah law was abandoned in favor of French civil law in 2010, they've become conciliators embedded in the French justice system. They conciliate parties to prevent a lawsuit or trial.
Mayotte and Réunion is the "happy face" of Islam in France. In Metropolitan France, with Maghrebans, Turks, Chechens, Islam is far, far more problematic, to say the least...