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 German*Ludwig*) or second names (Garnier from the German Werner)