Upvote:1
Like other commenters wrote, the transition from Latin and Old Frankish to the Langues d'oïl was progressive but, the first text considered to be written in Old French are the Oaths of Strasbourg (842).
So, even though the concepts of “French” and “France” were not defined at that time, you could say that Charles the Bald (823-877) was the first French-speaking king.