I'm really a layperson when it comes to the history of Europe, given I've only just started reading into it. It's just something I was thinking of when looking at a book that features a sequence of maps of European countries in history. After the Western Roman Empire dissolved, Germanic tribes seemed to conquer the majority of Western Europe, the Franks especially seemed to rule for some time, so why didn't german-based languages take over?
There are at least two important points. First of all, Germanic incomers were few in numbers. It is usually thought that Frankish settlement in Gaul must have been in the range of the tens of thousands, on a Latin speaking population of several million (common estimations propose 5 to 6 millions in Gaul). These settlers concentrated north of the Seine, where their input is still visible in toponymy, while they seem to have been very few south of the Loire. That being said, language replacement caused by élite take-over did happen; for instance, it seems that northern England (Northumbria) never was thoroughly settled by Germanic incomers—in fact, kingdoms there probably retained their native names—but it never the less ended up speaking Old English.
Here comes the crucial factor: Latin remained a language of power in Merovingian Gaul (and Wisigothic Spain, and Ostrogothic Italy). Gallo-Romans, educated in the Late Roman fashion, coming from families rooted in the most Romanised part of Gaul (Aquitaine and Provence), had a very important role: they formed the bulk of the episcopacy (which had an important political role on top on its religious cultural power), but they also contributed to the lay administration. For instance, a certain Asclepiodotus drafted laws for two Frankish kings in the late 6th century; people like Dynamius of Provence (roughly in the same period) corresponded in flowery style with Frankish administrators of the eastern court and obtained important positions. It seems clear that from a cultural standpoint, Latin won the battle. There might have been some resistance: though we don't have evidence for it in Gaul, we know that the Gothic élite (the dokimoi in Greek) objected to the classic education of their princes, in the early 6th century. However, this very situation shows how powerful the attraction of Roman/Latin culture could be for “barbarian” princes, and probably many other members of their “aristocracy.”
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