"Germanic" is kind of a tricky word because it might implies different things, notably politically and linguistically.Franks, as most of Barbarians the Late Roman Empire was confronted with, were a coalition of various peoples living along the limes (essentially Germanic tribes living in the regions since the Ist century BCE to Ist century AD : this coalition seems to have been as other more or less lax depending on the situation but essentially tied to their relation with the Empire.

While Barbarians already traded, entered in military service, worked for, raided Romans already before the IIIrd century; the crisis Rome knew at this point and the necessities to reinforce their armies in their wars against Sassanians led to a significantly weakened European borders, including the Rhenish limes.Franks raided Gaul repetetively during this period, but as soon as the late IIIrd century as the Empire recovered from the crisis and until the Vth century, something happened that significantly blurred the cultural distinction.Not only Franks, by the virtue living close to a political, economical and cultural superpower were already influenced by Roman features; but harbouring Roman slaves, deserters or fiscal refugees. Furthermore, either as defeated warbands or trough treaties, Franks were settled in Roman Gaul under different status, Salian Franks obtaining the status of federates, and several of their leader reaching the upper stairs of western imperial militia (consulate, military high-command, vice-emperor de facto,etc.).Basically, Franks became more and more Romanized since the IIIrd century, to the point it's likely that a significant part of Franks (similarly to Goths or Vandals) never really spoke a Germanic speech by the late Vth, even before Clovis was baptised.

The collapse of imperial authority in Northern Gaul which happened between the 430's and 450's led to a change of situation: the old Rhine border became pointless as such and was more or less abandoned. The region was already garrisoned and settled by laeti and federates since the IVth century and might have been more Germanized than the rest of the two Gallic dioceses. On the other hand it allowed Franks, already in contact with regional elites, to reinforce these ties and appears as a legal authority, whom rulers were both king of Franks and Roman generals.

As Clovis conquered most of Gaul in the late Vth and early VIth century, we can spot the appearance of a Frankish aristocracy beyond the original Frankish territories of Toxandria and Rhineland, with graves of chiefs harbouring all the signs of "Frankishness" : ring-swords^(1), beer, "ethnic" jewels, weapons, horses, some runes, etc. All things that look Germanic.Or...does it?

Jewels themselves were either made at Constantinople or they were made with material from there; runes' literal meaning is clear but not necessarily in context (one sword at Saint-Dizier being labelled "ALE"), beer-recipients was completed with wine-recipients, weapons are the same than used in Late Imperial period. Eventually, what's particularly interesting is that such grave display are unknown in Germania, where cremation was the rule.

It's quite possible that a significant part of the Vth and VIth centuries Frankish aristocracy was Roman in origin, or thoroughly Romanized, and stressed their Frankishness in a troubled period to both secure and reinforce their power.This movement would only continue up to the VIIth century, with people claiming to be Franks to avoid taxation, to be closer to the new political courts or simply to be fashionable : people we know being part of old senatorial families even began to adopt Frankish surnames out of nowhere. In the end, everyone north of Loire river was considered and considered themselves as Frank.

In this evolution, Frankish language (assuming there was one "national" language and not a set of Germanic speeches) was quickly abandoned as an institutional and everyday language in most of Frankish Gaul, except as a ceremonial use (presence of Germanic words in Lex Salica, itself largely a Roman law, is as much a proof of a wide survival of Frankish as an everyday language than a lawyer using Latin in court is a proof of its everyday use outside a tribunal).

Eventually, the answer might simply be "Franks didn't shift languages, Romans shift identities". Being Frank was never about the language, and essentially about acknowledging the Frankish king as your own.

However, Frankish linguistic influence is much stronger in Gaul than Gothic or Lombard in Italy and Spain : Old French morphology owes certainly a lot to a "Frankish accent" (which might be genuinely Frankish, or simply an already developing Northern Gallic accent; probably both). It doesn't says much, giving Germanic influence in Romance languages is negligible beyond lexicon, but Gallo-Romance languages, after Retho-Romance, are distinguished by this influence.

Another particular development is the Germanization of Rhineland : the linguistic border between Romance and Germanic languages is significantly set beyond the Rhine compared to the old border. It could be explained by an higher concentration of Germanic-speakers since Late Antiquity, settled as laetic or federated communities, effectively dominating an old military border region which depended more or less from the existence of the Empire (not too dissimilar, but in a much less important way to Illyricum). As such, the identify shift in these regions might have took the form of a linguistic Germanization (while culture mostly remained post-imperial) especially on contact with populations that remained largely Germanized in Frankish Germania, as the political/cultural border of the Rhine was no more.
However, the linguistic border took time to be set and as late as the XIth century, the survival of Mosellan Romance hint at a much more blurry regional distinction that the political stabilization of the post-Carolingian world might have clarified.

^(1) Frankish kings being "ring-givers", meaning that their trusted men were given a ring to be put on the sword's pommel to highlight a symbolic chain between the king and his fellowship. The Merovingian king being thus a "Lord of the Rings", maybe one of the first if the custom found in England and Italy could be attributed a Frankish origin.

- Les seigneurs des anneaux, Svante Fischer Jean Soulat, 2008

- Les Origines franques Vè-IXè siècles; 1990

- Shifting Ethnic Identities in Spain and Gaul, 500-700; Erica Buchberger; 2017