While the topic is wide enough to provide for more discussion, you might find some first elements of answer there.

Really basically, the German-ness of Franks was more about a social identity as a militarized people than linguistic : Franks themselves as a people were importantly under the influence of Roman civilization, emerging on its margins and integrated into its institutions since the IVth century.

As Franks tardily established their hegemony over post-imperial Gaul, where the Roman state collapsed but not its transformed institutions; they quickly integrating and intermixed with a Gallic aristocracy adopting its social codes as proximity with the Barbarian state was the key of perpetuating their social status with the disappearance of the Imperial state, at least in Northern Gaul : it's extremely likely, for instance, that Charlemagne's ancestors were issued from Roman lines, just as many Frankish aristocratic families. Already by the VIth century, it's difficult to make a difference between a Romanized Frank and a Barbarized Roman; the difference itself was probably not that relevant to them : by the VIIth century, virtually everyone North of the Loire was considering themselves as Frank, regardless of origin.

Eventually, with the exception of Rheinish regions where settlements by Germanic-speaking peoples since the IIIrd and a complex and slow process of germanization was only firlmy distinct from the Romance ensemble by the XIth century, Romance language was "only" diversely influenced by Germanic elements in both vocabulary and morphology.