I cant speak on the validity of the 20% comment , however, french became the language of France during the French Revoloution. Before French borders became much more realized during the end of the 18th century, France was composed of french, flemish, german, italian, among other languages. By 1795, the national convention instituted state funded schools, part of the doctrine was that all schools be taught only in french. By the time Napoleon came to power, french was seen as a national language in an increasingly nationalistic country.

Hi, you may be interested in a couple of related posts

• I heard somewhere that what we consider French is essentially Parisian and even up until the late 19th century most people in France spoke a different language at home. Is this true? - a long discussion thread that includes comments on several dialects. The post is still open (not archived) if you want to join the conversation

• Did southern France mainly speak Occitain languages in the past? - a closer look at the Occitan dialects of southern France by /u/idjet