How long were Old Frankish dialects spoken in modern France?

Question

How long were the Germanic dialects commonly subsumed under the term "Frankish" spoken by Frankish people in Northern Gaul, and how long did it take until they were completely supplanted by the Romance dialects that would later evolve into French?

EDIT: I'm aware that Alsatian and Dutch are still spoken today, but only in parts of the country that were either not part of France until historically recently or had never had a Romance-speaking majority even before the fall of the Western Roman Empire or the defeat of Syagrius by Hlodwig. I am talking about the Frankish idioms in regions where Franks were a minority.

Answer 1

I don't think we have an ironclad handle on that. This period was about as dark as the dark ages got.

Its likely the Franks in Neustria were never more than an elite ruling class over a population that remained overwhelmingly ("Vulgar") Latin-speaking.

It appears fairly certain that Charlemagne (740's to 814) spoke Frankish, but when his grandson Charles the Bald (875-877) exchanged oaths with Louis the German (using each other's languages), the latter used Latin/Old French.

According to Holmes' A History of the French Language, Frankish was probably still used for some official purposes up to about 850, and disappeared as a spoken language from ex-Roman Gaul sometime in the next century. I'm not sure how well-accepted that is.