The topic surpsiringly comes up a lot, but you might find some elements in an answer I proposed to a smiliar question, altough more discussion and elements are certainly welcome.

"German-ness" of Franks was eventually more rooted down as a social identity as a militarized people on the "integrated periphery" of the Roman Empire than really linguistic. : Frankish language(s) themselves being poorly attested outside latinized forms, altough influential in the making of northern Gallo-Romance languages.

Frankish hegemony being set in a Gaul where the imperial authority had collapsed without provoking the ruin of Roman institutions, a Barbarian state and society intermixed and integrated within provincial frames and with a Gallic aristocracy that began to adopt its social codes while maintaining markers of romanity as soon as the early VIth century; where distinguishing a romanized Frank from a barbarized Roman is both difficult and probably not that relevant to begin with further provincial elites in Gaul were integrated into the framework of the Barbarian state. A lot of Frankish aristocratic families of the late Merovingian period had likely Roman ancestry, including Carolingians; and by the VIIth century, virtually everyone North of the Loire (exception made of Brittany) considered themselves as Franks (altough it wasn't necessarily exclusive to regional identities; e.g. Neustrian, Burgundian or Saxon)

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.