I’m neither a historian nor a linguist… so not authority on the subject, I only have schoolish memories….but my first impression is that you think that the Franks were the ancestors of the French people and of the French language.
Well, there is more to it!
The Franks were a tribe that occupied regions of northern Europe (regions now in Germany, the Netherlands…). At one point in history, the Franks invaded the northern part of Gaul and then conquered the whole of Gaul, and Gaul had Frankish kings (Clovis was the first in the 6th century), but the Frankish language (German) never really took among the masses.
The inhabitants of Gaul at the time spoke a mixture of Gallic (a Celtic language) and latin (Gaul had become part of the Roman Empire, circa 200 BC).
What you read is correct: the Franks were a Germanic-speaking nation, but the Franks are not the ancestors of the French, not to speak of their language!
The Frankish language – German – has given German and Dutch. The name France was adopted because there have been Frankish kings but the Franks did not give France their language.
My generation (well over half a century ago…) learnt at school about “nos ancêtres les Gaulois” (The Gauls, our ancestors…) never “nos ancêtres les Francs”! The Franks were the naughty invaders.
Obviously nowadays France is a meltingpot and the French language a mixture of a lot of languages… but the basis is Latin with probably some traces of Gallic, traces of Frankish/German, large chunks of English….
Trouvé dans Histoire d’une langue : le français de Marcel Cohen (1973)
In Old French there were over 400 words on Frankish Geman origin, but most of them did not survive.
Les mots d’origine germanique ou au moins influencés dans leur forme par le germanique désignent naturellement des institutions ou des objets de même origine ou ayant une importance plus grande dans les milieux germaniques. Citons entre autres marche et le dérivé marquis, baron, chambellan, guerre, trêve, fourreau, heaume, remplacé plus tard par casque, emprunté à l’espagnol, fauteuil, guetter, saisir, garant, épervier, aussi des temes ruraux : blé (peut-être mélangé avec une racine celtique), jardin (allemand Garten) et des adjectifs de couleur et autres : bleu, brun, sale. Des Gallo-Romains ont pris des noms germaniques d’où l’abondance maintenant de prénoms comme Louis (allemand Ludwig) et de familles comme Garnier (allemand : Werner)
Words of German origin, or at least influenced by German, designate institutions or objects which are more important in German populated areas.
Some Gallo-Romans adopted took first names form the German (Louis from the GermanLudwig) or second names (Garnier from the German Werner)
The territory that is now France was formerly Gaulish-speaking. After the Roman conquests, vulgar Latin slowly became the dominant language. Only a small part of the population switched from (an ancestor of) German to (an ancestor of) French. There was a major shift, not from German to French, but from Gaulish to (proto-)French, roughly going on from the 1st century BC to the 5th century AD.
Latin was the trade language (having supplanted Greek in the west of the Mediterranean), German was only the language of the military and rulers. Latin furthermore had an established writing tradition, and so was for a long time the language of legal documents (in France, until 1539). I think these is the main reasons why Latin managed to supplant Celtic languages in a lot more areas than German supplanted Celtic or Romance languages.
The 842 oaths of Strasbourg is often considered to be both the oldest extant Old French text and the oldest exant Old High German text. The language of the French version is recorded as “romana lingua” (and the German version as “teudisca lingua”) in the Latin preamble. This early stage of Old French is termed Gallo-Romance, specifically here an oïl dialect, of which a subdialect would later dominate other dialects in France.
The Franks were once our rulers but definitely not our ancestors… Because they have ruled our lands and created the roots of “French” monarchy the name “France” stayed until today to describe the country. The descendants of the small Frankish ruling class was soon obliged to use mainly the local Latin-based languages (not only modern French but a lot of other variants, Oil languages and Occitan variations). As soon as they lost their language they were not really Frankish anymore. They were not numerous enough to be able to impose their culture to the Romans of Gaul. This is not was happened in Britain, were the Anglo-Saxons have replaced the former British culture, probably because Britain was much less romanized than Gaul (much farther from Italy, while Gaul is neighbouring it and has been Romanized since a much longer time), and also because the Anglo-Saxons were probably much more numerous than the Frankish invaders.
Also, the Frankish had been living for a long time in contact with Roman civilisation and they wanted to be part of it, not to destroy it. This also probably played a role in their cultural integration into the Roman culture of Gaul.
The ruling class started then to integrate into the aristocracy people who were not from the “Frankish stock” and mix completely until being really indifferenciable.
That said, even today, when looking at the people descending from former aristocratic classes, it is not unusual to find more people with slightly more “Nordic” looks than in the general population, especially when compared to the people south of the Loire.
It is difficult to say how much DNA is coming from Frankish people in modern French population.
Our language ‘French’, is probably badly named in the sense that it gives wrongly the idea that it refers or descend to the Franks or to the Frankish language when it does not at all. Maybe it would have been linguistically better described as “Galloroman”, even if it is only one among many other Gallo-Romance languages (Wallon, Picard, Gallo, Piemontese, Arpitan, Lombard, Gascon, Provençal, etc.)
But it has been called that way for centuries and we should not take the etymology too strictly assuming things that are not true.
There are a lot of words in modern French that come from the Frankish languages, more if you consider northern dialectes.
However the Salian Franks, who lived in what are now Belgium and the Netherlands before they progressively moved to France, quickly began to speak Latin, and Gallo-Roman dialects.
Some Franks spoke Frankish languages which are much closer to modern German than to modern French or even to modern Flemish (for example Charlemagne).
The Franks were not the only Germanic tribes that settled to France after or during the Barbaric invasions. Allied to the Romans and Gallo-Romans under the Roman empires, they progressively came to southern Germany, then to France during the IVth and the Vth centuries, often serving in the Roman army and auxiliary cavalery, before settling.
At this time there were still a lot of languages in France, not all of them were celtic languages, so the only widely used language was Latin, and later a blend of local Celtic languages, Frankish or Germanic languages known as the lingua franca.
The Franks were only some of the people that are the ancestors of the modern French (and Belgians btw), like the Wisigoths (who later moved to Spain), the Celts, or the Burgonds.
Up to 10% of the French language is derived from Frankish root words – Nadeau, Barlow, The Story of French, pg 24.
Frankish is not the only source of Germanic words in the modern french language. Other Germanic words were picked up along the way from the Goths, Burgundians, Normans (after they brought back some English words of Germanic origin following their invasion of England) and even some Germanic words that entered Latin before Gaul was conquered by the Romans. As is always the case history is complicated and does not lend itself easily to simple answers…
Also, as the ruling class the Franks had a major say in which version of “vulgar Latin” was ultimately selected and spoken by those in power at the time. This was the version spoken in what is now North Eastern France and they modified it due to the accent they brought which included Germanic stress timings, syntactical elements and led to the unique character of the Langue d’oil where Frankish settlement was heaviest. Ultimately French is considered the most germanic of the various vulgar Latin dialects that evolved following the collapse elf the Roman Empire – e.g. as opposed to Spanish, Portuguese and Italian which remain far “melodic” in sound by comparison without the Germanic influence on their evolution.