Development of Romance languages...

 The history of the French language is very complicated, first it's not the Parisian French we speak but the Tourrangeau one, that was spoken in Tours.

This is why today the purest French is considered to be from Tours.

First, you have to know Gaul was not homogenous at all back to the pre-roman era.

The main group were the Gauls, Brennus, Vercingetorix and so on. They spoke a celtic language which was a Gallic language, the Gaulish one.

http://http://www.orbilat.com/Encyclopaedia/G/Gaulish_langua ge.html

This page show an overview of it, as you can see it was fairly close to Latin already with some differences though. Aside of the Gauls the southwest bank of the Garonne River spoke Aquitanian, a now extinct Basque related language.

The Roman colonies like Narbonne spoke Latin and the Greek colonies like Marseille spoke Greek.

Under the Roman Empire, the Greek colonies switched to vulgar latin while the gauls would adopt it fairly easily too due to the similarities, the Aquitanians would retain their non indo-european language for a longer time.

In the 5th century, the Aquitanians started to devellop their own Romance language called Gascon. But it came from the Aquitanians to the latin so it has a very distinct spelling and pronounciation. The southern languages, Languedocian, Provencal and Catalan develloped from one side, Corsican also develloped as an Italo-Romance group.

In the north, the Germanics left a bigger influence on the spelling.

http://www.absoluteastronomy.com/encyclopedia/o/oa/oath_of_s trasbourg.htm

The Oath of Strasbourg between Charles the Bald and Louis the German is remembered as the first text in old French, and the first text in a distinct romance language to Latin. It's writen in both Old French and Old German.

The Breton would settle in Brittany, comming from the British Isles and Germanic would settle in large amount in Flanders (Nord Pas de Calais), Lorraire (Metz and Nancy) and Alsace, there the Germanic languages would prevail as the Flemish, Frankish and Alsatian languages.

Then, in the middle ages, we could put this distinction:

http://www.tlfq.ulaval.ca/axl/monde/langues_celtiques.htm

There you have a good desciption of the Celtic languages. Apart the Gaulish about little is known.

The big French group could be descripted as heterogenous, people in Paris had a strong accent, while people from the Loire Valley had a "flatter" one. East of the Loire Valley, Gallo develloped as a mix of French and Breton.

Poitevin and Saintongais had some features of the Occitan Group, Champenois and Picard had some minor differences too. Norman had a slightly different spelling and some Norse vocabulary, especially about maritime term... All this big French group was mutually intelligible without problems most likely.

Franco-Provencal is a subgroup having features of both French and Occitan, it was spoken in Savoy, French speaking Switzerland and Aoste in Italy.

The Occitan group was characterised by the word used for "yes" that was "oc", in contrario of the "oil" used in northern France.

This is a rather extensive map, I develloped Gascon, most of those languages develloped more or less the same way from vulgar latin apart Gascon.

The Basque is a direct descendant of the Aquitanian language. It's not indo-european.

Corsica had an Italo-Romance language, sounding like the one spoken in Sardaigna.

Catalan is fairly close to Occitan and develloped nearly the same way until the rise of Catalan sensibility proclaimed it apart of Occitan.

Frankish, Flemish and Alsatian are Germanic languages with a different history.

Finally, Fran�is I King of France, made French of Tours the language for administrations and parliaments.

Richelieu founded the Academie Nationale to regulate it.

Finally, it was decided everyone should speak it and adopt it.