"the French language has been essential to the concept of 'France', although in 1789 50% of the French people didn't speak it at all, and only 12 to 13% spoke it 'fairly' – in fact, even in oïl Language zones, out of a central region, it wasn't usually spoken except in cities, and, even there, not always in the faubourgs. In the North as in the South of France, almost nobody spoke French."
Eric Hobsbawn, Nations and Nationalism since 1780 : programme, myth, reality (Cambridge Univ. Press, 1990) chapter II "The popular protonationalism", pp.80–81 French edition
Renée Balibar, L'Institution du français: essai sur le co-linguisme des Carolingiens à la République, Paris, 1985 (also Le co-linguisme, PUF, Que sais-je ?, 1994
Renée Balibar and Dominique Laporte, Le Français national: politique et pratique de la langue nationale sous la Révolution, Paris, 1974
Edit : Just found another piece of data : Balibar estimate to 25% of people in France who could speak French in 1789 and 75% in 1800.