The first Holy Roman Emperor Charlemagne (Karl the Great as he is known to the Germans) was not French! The Franks were a Germanic tribe originating east of the Rhine from what is modern day central western Germany. Charlemagne’s native tongue was the old high German dialect, which was also the language used at his court in Aachen.
The first document to appear in what can be termed the old Gallo Romance ancestor of ‘French’ (the oaths of Strasbourg of 842) was not even written until nearly thirty years after Charlemagne’s death.