Origins

It is widely admitted that the French language emerged from
the vulgar Latin of the time. It is complicated to express the actual
truth of the relationship, because language is what we communicate
with. Who is to say how Latin itself came to be formed.
This is also all the more enigmatic because the first written
document in actual French is the transcrition of an
oath of mutual defense between two brothers against a too
ambitious third. Each recites an oath in the language of the
other, and to make sure everything is clear, there is a separate
oath for the men of each brother, in their own language. Now,
that's a deal!

All three brothers are grandsons of Charlemagne, 'roi des Francs...'

For linguists, the Serments de Strasbourg are a rich find because we have
the same general idea/thing expressed in two different languages, one 'romane'
and the other 'germanique'(from germanus, of the same germ). 'Latin' comes
from 'Latium', and the ancient goddess Lati (liquor, or lust!?).

source: Wikipedia

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Below, the city of Strasbourg (and my current wallpaper. A very charming place,

and the whole region has wonderful pastries...):

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By medieval times, there is a distinction made for le français ou françois

between langue d'oïl in the North and the langue d'oc in the South. The first

to use this distinction is believed to be the Italian poet Dante (1265-1321), who had

a third langue di si for nascent Italian.

Oil  itself is what later became oui (yes), and brings together the latin hoc (this) and

sic (thus).

The langue d'oil came to predominate over the French territory over time,

while the langue d'oc retreated to Spain. The first is more Germanic than the 

second and has Celtic elements.