I don't know alot about germanic migration between 100 and 500 AD but always found it particularly odd how the vandals, visigoths, ostrogoths and Franks who settled most of western Europe had their language evolved from old Germanic to the romance languages of today such as italian, french, Spanish, Portuguese etc. So what really happened? Was it because of christianity or was it the remaining roman inhabitants that caused it?
An answer (Why are the spanish and french not considered Germanic; u/Libertat) I posted some time ago might provide with some elements of response.
Basically, it's doubtful that each Barbarian coalition had its own "national" language, being made up from the IInd century of various (indigenous or migrating peoples) and being growingly Romanized due to their relations with Romania and the influx of slaves, fiscal refugees and deserters among them from the IIIrd century to the Vth, especially as Barbarians entered the empire. A good part of Goths, Franks, Vandals, etc. probably didn't really spoke a Germanic language at this point, while Germanophone Barbarians were more or less importantly Latinized
So, it's both a result of the resilience of Late Roman society (or its collapse in Britain and Illyricum) and the appearance of Barbarians as "peoples of the limes" then as Roman armies/groups since the IVth century.
Not that they didn't had an influence on the development of Romance language (although this is especially the case for northern Gallo-Romance and Retho-Romance languages), neither that the Germanization of the Rhineland couldn't be attributed to a stronger Germanophone presence : but we're talking of an evolution that took centuries and didn't really stabilized before the Xth century. Apart from that, actual evidences for an everyday use of Germanic speech in former western provinces are quite rare, and generally assumed to have died out as such (although preserved in ceremonial and institutional terms) by the Vth or VIth century.