It was said among Army recruits that only 20% could speak the actual French language. The French language itself was codified around 1800 based on the Parisien dialect, spoken around Paris. So modern French is just Parisien the same way that modern German is just Upper Saxon and modern Italian is simply Florentine Tuscan.
What were the rest of the soldiers speaking? Many of them may have been speaking patois. Patois are generally other langues d’oil, related to Parisien. There are many of them, but they are dying out. In general, patois are not intelligible with Standard French.
Many also spoke Occitan, a language between Spanish and French spoken in the south of France. Further, some Occitan dialects are hardly even understandable to other Occitan speakers. French speakers are quite lost when listening to an Occitan speaker.
130 years ago, there were probably many speakers of Breton in Brittany. Breton is related to Welsh, and a French speaker can’t understand a word of it.
Surely, there were many speakers of Basque in the southwest of France. Basque is incomprehensible to a French speaker.
In far northeast France, Flemish is still spoken, and it was much more spoken 130 years ago.
In the part of France near Luxembourg, varieties of German are spoken, Moselle Franconian, Lorraine Franconian and Luxembourgian. These are actually three separate languages. They were much more commonly spoken 130 years ago.
To the south, Alsatian was spoken in the Alsace Lorraine. A traveler to this region wrote that in some areas people speak German, in others they speak French, and in others they speak some language that is neither German nor French. Alsatian is a German dialect that is declining. But it was very widely spoken 130 years ago.
In the far southeast of France, Nissart, Monegasque, Montenasque, and Intermelian are spoken. The last two are dialects of Ligurian, a language spoken in Italy. The first two are Occitan dialects with a heavy Ligurian mixture. All of these were spoken much more 130 years ago.
In Corsica, Corse is spoken. Corse is related to Standard Italian. It is declining, but was widely spoken 130 years ago.
In the area near Switzerland, a language called Arpitan or Franco-Provencal is still spoken. It was much more widely spoken 130 years ago.
In the far southwest of France in Rousillon, Catalan is spoken. It is dying out, but was probably widely spoken 130 years ago.
As you can see, the notion that only Standard French is spoken in France is quite mistaken. It was even less true 130 years ago, when only 20% of the population spoke the standard language.