As a Frenchman, I don't understand how northern Gaul could have been "central germanic" by the time of Charlemagne.
Since Franks invaded north-eastern Gaul, they have constantly interacted with Latin speaking Gallo-Romans, converting to their religion and adopting their morses, to the point that there were soon no more differences between the two cultures. Gallo-Romans called themselves Franks, and Franks spoke Old French, a germanized Latin language.
In that regard, Frankish culture of the eighth century shouldn't be described as central Germanic, as it is in the game, but as Latin.
For more about this assimilation process, you can read German historian K.F. Werner.
Since Franks invaded north-eastern Gaul, they have constantly interacted with Latin speaking Gallo-Romans, converting to their religion and adopting their morses, to the point that there were soon no more differences between the two cultures. Gallo-Romans called themselves Franks, and Franks spoke Old French, a germanized Latin language.
In that regard, Frankish culture of the eighth century shouldn't be described as central Germanic, as it is in the game, but as Latin.
For more about this assimilation process, you can read German historian K.F. Werner.