There's no much trace that population spoke Vulgar Latin in most of these regions in the Vth century : Raetia (like the whole of Illyricum) was essentially romanized in urban and military centers, and with a really limited population. It's to be remembered that a good part of the Roman population was evacuated in the Vth century.They also changed Romance languages in the whole of Austria, German Switzerland, Southern Germany, Rhineland, likely Flanders, England
Switzerland and transrhenan Germania are a different case, but generally speaking weren't that deeply romanized either and more early took over by Alamans. We indeed have few reasons to believe that Celtic speeches were definitely present there by the Vth century but if it did managed to survive in Gaul and Anatolia, it probably did there too IMO.
Rhineland was definitely harbouring Romance communities up to the XIth century, whom better known part was the Mosellan Romance. The linguistical translation took time and was really not stabilized by the end of Late Antiquity : the appearance of strong political/cultural centers in Germany really helped there, altough the absence of real structural romanisation past the Rhine certainly helped the local population to maintain their speeches. Note that we know that Gaulish language was still practiced in northern Gaul by the Vth century in the Rhineland specifically.
Flanders, because most of it was poorly inhabited for centuries, with literal abandonment of land. The settlement of Franks and other western Germanic populations, with a similar situation than Rhineland except with a much lesser post-imperial romance presence is to be taken in account.
There, compared to some other regions, the permanence of a strong trade (North Sea trade, notably) with Germanic communities in Britain, Scandinavia and northern Germany probably represented a dynamic support for maintained germanisation.
And, least but not last, there is simply no trace of Vulgar Latin used by Britton population in the Vth century : Brittonic speeches were certainly largely dominant. Note that the British case is quite special compared to the mainland, as in Roman structures were utterly ruined in the island (not due to Germanic settlers, than Gaelic raiders and the general collapse of Roman society in a relatively poor and isolated Britain), which was not the case in most of western Romania.
Linguistic germanisation of the regions you mentioned are very factual events, but it did took more time, was more gradual and less one-sided than you suggest it was. Note that it could mean, with different circumstances, that more regions could be germanized, altough it'd probably restricted to either places with few post-imperial features, either socially/institutionally ruined, if not both.
Preservation of late imperial structures in Italy (or post-imperial in Spain and Gaul), meaning the sheer social-cultural weight of Romanity, while Illyricum was poorly inhabited, poorly touched by Roman institutions and society safe in relatively isolated and small cities which didn't remained a thing very long after evacuation and unability of the Roman state to support what ammounted to a military border (hence why foedi were generally sent in this direction).What allowed language change Romance to German in Danube valley, but not Po valley?
This romanity did not just mattered to local population, but as well to Barbarians whom very ethnogenesis implied a deep romanisation (would it be only due to the important integration of provincial populations since the IVth century).
Destroying Roman structure and culture in Italy, essentially.What would enable language change to German in Po valley?
Something that neither the continuous crisis of the Vth century, neither the particularly destructive Gothic Wars and the subsequent Lombard take-over managed to do. I don't know personally what could realistically pull this kind of destructive event (which would probably have consequences on Barbarian own social/political integrity anyway), as we're talking of the literal center of late-imperial/post-imperial Romania in western Europe , for pretty much obvious reasons.
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