The issue there is that, when it come to Franks (or vritually any Barbarian people settled in the Late Roman Empire), there's the question on how Germanic they actually were, and the bundle of evidence rather goes to answer "not that much".

Contrary to Germanic groups that settled in a ruined eastern Britain in the Vth century and while mixing with the remaining Britto-Roman population were essentially "newcomers" into the Late Roman sphere; mainland Barbarians were deeply into contact with it since their emergence in the late IInd century as coalitions of border people.This contact took the form of trade, political and military exchanges or service as much as there were raids and conflicts; and not only Barbarians integrated important numbers of Roman deserters, fiscal refugees or even slaves (either taken by Barbarians or fleeing to Barbarians), but Barbarians themselves were importantly integrated to Roman institutions especially as they were settled in Roman provinces and as, forming autonomous armies, took the political lead by the Vth century, further integrated Roman elites changing political identities (serving directly a Barbarian king rather than the Roman state is pretty much the definition of a Barbarian).

Such romanisation had pretty much obvious consequences for most Barbarians who largely adopted, or more accurately for many of them, kept Late Roman featues : there is no evidence that Goths or Burgundians kept using a Germanic speech as an everyday language by the late Vth century, except for symbolic and ceremonial use.

Franks might be considered a bit a part at first glance : their kings didn't converted before the turn of the VIth century, and it's only with Clovis' reign that they took over a large part of former Roman Gaul. But this is partly a trompe l'oeil : while Salian Franks took the primacy in Gaul only in the late Vth/early VIth, Franks were already settled in northern Gaul, beside Salian Belgica and Rheinish kingdoms, since the IVth century with many imperial high officers or commanders being Franks (Merobaud, Arbogast, Bauto, etc.) : the transition from Merovingians as petty-kings of Belgica Secunda to overlords of Roman Gaul was thus much smoother (especially as the military constitution of Clovis' realm was importantly based on taking over Barbarian regional armies settled in Gaul).

Frankish traditional beliefs, that said, are a bit more attested for the late conversion and their maintained legality until the mid VIth century and giving there wasn't a great Christianisation effort for Frankish Germania before the later Merovingian era. But a bit more attested compared to the lack of evidence on other Barbarians isn't saying much and references are objectively limited while, as much as it can be told, there weren't a great departure from contemporary pagan practices among continental and insular Barbarians

Sacrifices of horses and pigs are attested in both archeological and historical sources (the theft of animals set for sacrifice was punishable by an important fine in the Salic Law); bulls might have been considered as a ritual animal; and it's likely that early Merovingians claimed a divine ascendancy, at least the curious story accounted by Fredegarius about Merovech being issued from a sea monster and a Frankish queen would lead us to believe it.It is possible that they as well practiced human sacrifices, but safe a possibly calumnious accusation by Procopius, there's no real evidence for that.

As for sanctuaries, contemporary authors name them fana, temples (implying buildings, quite possibly made of wood considering clergymen or Christian queens regularly tried to set them on fire) without further description.

Such practices continued to exist along Christianism until the mid-VIth century, but were rather "rustic" (even if the Frankish king tolerated and protected these, especially if made on his behalf) and there's no big evidence that Frankish nobility remained pagan, rather the contrary : not only several of them intermarried with Roman families as soon as the Vth century (contrary to other Barbarian laws, there wasn't an interdiction on intermarriage for Franks) with some important families (such as Arbogast of Trier's) converted before Merovingians; but while the royal conversion didn't implied the conversion of the Frankish people, it did implied the royal leudes (roughly, his aristocratic entourage or people part of his royal court) had to, eventually.

While the transformation of a christianized state into a Christian state took time, by the mid-VIth to early VIIth century, Frankish kingship took a certain biblical outlook : the idea that it kept pagan features or pagan "charismatic magic" is dubious. An often mentioned exemple, the long hairs and beard of the kings being the mark of their semi-paganism is rather due to a Carolingian historiography bent on disparaging the still popular Merovingians : while long hairs are a common Barbarian fashion in Late Antiquity (not necessarily more "magic" than the Suebian knot), their attested sacralization (it's likely Merovingian kings put some of their hairs into their official seals) is mostly tied with Biblical kingship and the bearing of David and Salomon, the models of the Merovingian kings.

More accurate magic rites can be found, however, in the Vth century with swords being marked of runic letters (one of the swords of Saint-Diziers spelling A-L-E) but their role or working out isn't really known. The practice itself seems to have quickly died out.

Barbarian rituals weren't necessarily religious, and the sword-rings found in Merovingian era might be of particular interest there : the focus given on rings in Beowulf, who gives or doesn't gives them to their followers (the former being generally good kings, the second bad ones) gave birth to an important trope that was echoed into Tolkien's Lord of the Rings.While not necessarily the only form of rings given away (sealing rings given by the kings is a possible and attested alternative), rings set in the pommel of swords are found as soon as the Vth century in Gaul before they were found in England or Italy : such rings were given to trusted leudes (either one or two rings tied to each other) as a symbolic bind between the king and them. The practice was common enough for the Merovingian king to likely be the original Lord of the Rings before it got widespread, due to Frankish hegemony in Northern-Western Europe, in Anglo-Saxon England or Lombard Italy.

Similarly, what were called the "malbergic" parts of the Salic Law (from "the assembly"), Latinized semi-German words and expression into Frankish law, do attest of a certain status of Frankish language (or rather, Germanic dialects spoke by Franks, as there is no real trace of a single "national" language) even as it disappeared as such in the early VIth century, at least in most of Gaul.

All of these, that being said, took place in an essentially Roman society : no more than the bearing of new names by Romans (a noble named, as many of his ancestors, Lupus ending up as naming his own son Wulf, for example), having a sword-ring or displaying weapons in one's grave was seen as necessarily "Germanizing" but adopting the new habits and displays of the time. Most of the fashion, weaponry, institutions, etc. and eventually cultural horizon of the Franks was Roman. Their own history as they told it was Roman, rather borrowing from classical and biblical references than anything else.The Late Ancient and Merovingian period in Gaul actually witnessed the blossoming of Gallic literary tradition and the lot of histories and chronicles of the Franks (Gregory, Fredegar, the anonymous of the Liber Historia Francorum) or the saints lives formed the bulk of narrative tradition of Franks.

Rather than Beowulf, there's the Frankish hagiographies with their ghosts and dragons; rather than riddles, the poems of Verentius Fortunatus, displaying the riches of Frankish courts in the midst of almost untranslatable puns (both because of the language's barer, both for sanity's sake) and eventually, what was one of the main genres in Gaul : history. This was not a Germanic folklore, of course (and even the Anglo-Saxon folklore and conceptions were influenced by a Roman substratum or adstratum) but this was part of Frankish culture.

- Cultures cléricales et traditions folkloriques; Jacques le Goff; 1965

- Les racines chrétiennes de l'Europe, Conversion et liberté dans les Royaumes Barbares, Bruno Dumézil, 2005

- Les seigneurs des anneaux ; Marie-Cécile Truc, S. Fischer, J. Soulat and L. Fischer; 2008