They were initially Breton but they were soon dominated by French culture from France and England (which was also French-speaking). The rulers of medieval Brittany were culturally French although the general population remained largely Breton.

The distribution of languages in medieval Brittany was a bit different than it is today. For the linguistic situation we have to go back to the 5th century. The Celts in the Roman province of Gallia spoke Gaulish, the form of the Celtic language spoken on the continent, while the Celts in Britain and Ireland spoke related, but not quite the same languages. By this time the Gauls might have even switched entirely to Latin, although the Celts in Roman Britain apparently didn’t adopt Latin, at least not to the same extent.

The Romans left Britain in 410, and Germanic tribes invaded/settled in both Britain and Gaul. The British Celts were pushed westward into modern Cornwall and Wales, but some of them also fled across the channel to Gaul and settled among the Romanized Gauls in the Armorica peninsula. Eventually Armorica was also called “Britannia”, since that’s where all the new inhabitants had come from.

So, the medieval and modern Breton language is closely related to Welsh and Cornish, not continental Gaulish. Welsh, Cornish, and Breton are part of the Bryttonic branch of Celtic languages; they’re more distantly related to Irish, Scottish, and Manx, which form the Goidelic branch. (Continental Gaulish was probably extinct before these different branches emerged.)

Like the Germanic tribes that settled in England, the Germanic Merovingian Franks also settled in northern Gaul and Armorica among the Romans (or Romanized Gauls) and the Breton settlers from Britain. In Armorica/Brittany there was a Merovingian aristocracy in places like Nantes and Rennes but they competed with/were opposed by the native Breton aristocracy. For the Carolingians who replaced the Merovingians, this was the “Breton March”, a far-off borderland, but they sent representatives (“missi” in Latin) to impose imperial authority there.

After Charlemagne died his successors were much weaker. They had to deal with invasions from Vikings and other invaders and generally left Brittany alone. Louis the Pious appointed the Breton nobleman Nominoe as his imperial representative in Brittany, but in 845 Nominoe defeated Charles the Bald (Louis’ son) at the Battle of Ballon, and formed his own independent state. This is sometimes called a kingdom, sometimes a duchy, sometimes a county, but whatever we call it, Nominoe is traditionally considered the founder of independent Brittany.

The Vikings or “Northmen” conquered some of the Frankish realm north of Paris and carved out their own state too, the Duchy of Normandy. But they invaded and partially conquered Brittany too in the 9th and 10th centuries, until they were defeated by the Breton king/duke Alan Barbetorte in 939. Alan restored Breton independence but by now, Brittany was connected to and heavily influenced by the French. The rulers of Brittany intermarried with nearby French nobility, especially with the families of the counts of Blois and Anjou, and the dukes of Normandy. By the time of Alan IV (duke from 1084 to 1119) the ruling dynasty of Brittany was more or less French. Alan IV was probably the last to speak Breton natively.

Through their connections with Normandy and Anjou, the dukes were also closely connected with the kings of England, and traditionally held a courtesy title in England (the earldom of Richmond). In the late 12th century Brittany was essentially just a battleground in the struggle between the kings of France and England - the English of course were also French-speaking at the time since the ruling dynasty came first from Normandy and then Anjou. So culturally and linguistically, England and Brittany were just as French as the the French kings. The major castles and fortifications in Brittany, including the Castle of the Dukes of Brittany in Nantes, were mostly created by these French-speaking dukes, who were either dominated by or appointed directly by the English or French kings in the 12th and 13th centuries.

The non-aristocratic population of Brittany still spoke Breton though. French chroniclers

“write in amazement about its inhabitants' strange ways…given to a life of fighting and horsemanship, neglected agriculture and good manners, lived on abundant milk but scant bread, and pastured huge herds on wide open tracts of land from which no other harvest was taken.” (Galliou and Jones, pg. 174)

This description likely isn’t totally accurate, because educated French writers liked to use ancient Roman depictions of the Gauls to explain contemporary Bretons, but still, it shows that they considered the Bretons quite different. The 12th-century philosopher Peter Abelard, who was from Brittany but was culturally French, said the Bretons were

“not only incomprehensible but uncouth in every respect” (Galliou and Jones, pg. 175)

Despite being dominated by England and France, Brittany remained legally independent until Duchess Anne married the French King Charles VIII in 1491; after Charles died she also married Louis XII in 1498. Their daughter Claude married Francis I of France and Brittany was formally incorporated into France through their children, but not until 1547. But Brittany still remained a distinct part of the French crown up until the Revolution, at least.

The boundaries of modern Brittany are different now - there is the modern région of Bretagne, with its capital at Rennes, and which consists of the départements of Ille-et-Vilaine, Morbigan, Côtes-d’Amor, and Finistère. But until the régions were created in the 1950s, Bretagne was also considered to include Loire-Atlantique (or Loire-Inférieure at the time), the département centered on Nantes. Loire-Atlantique is now in the région of Pays-de-la-Loire though. However, Nantes is still considered to be part of “Historical Brittany”. You’ll sometimes see “44 = BZH” graffiti in Loire-Atlantique (44 is the département code and BZH is the language code for Breton), there are unofficial “now entering Historical Brittany” signs when crossing into LA from other parts of Pays-de-la-Loire, and there’s always chatter about reuniting LA with Bretagne, but that’s probably not going to happen.

Everyone in Bretagne and Pays-de-la-Loire now speaks modern standard Parisian French since that’s what they learn in school. A very small amount of people still speak Breton, especially the further west you go. There are some Breton-language schools and organizations, and road signs often include Breton names (the sign when you enter Nantes also says “Naoned” for example). You might hear street musicians playing Breton music on Breton instruments, and you might see some people dressed up in "traditional Breton dress", just for the tourists. There's a statue of Duchesse Anne outside the castle in Nantes where she's dressed as a Breton peasant, even though she certainly wasn't a peasant and she was totally French in culture and ancestry. If I can give a personal anecdote, when I lived in Nantes I never heard anyone actually speaking Breton.

The other langue-d’oil you mentioned, Gallo, was originally the regional dialect of French as spoken in the northwest; all those dialects (Poitevin, Saintongeais, Norman, Picard… not to mention all the very different langues-d’oc in the south) were all stamped out thanks to the efforts of the national government to standardize language and education across the country. Well maybe not stamped out, but the regional dialects and languages were heavily discouraged at least; children used to be punished for speaking Breton in school. Some very old people might still speak Gallo but it’s very unlikely to hear that anymore these days either (again, just a personal anecdote, but I never heard anyone speak it in Nantes).

So in brief, medieval Brittany was at first Breton in culture and language, but was gradually dominated by French; the general population continued to speak Breton for centuries, but the aristocracy spoke French, or the particular northwestern dialect of the langue-d’oil that developed into Gallo. But now everyone speaks standard Parisian French just like the rest of the country, thanks to government language policies in the 19th and 20th centuries.

Sources:

Judith Everard, Brittany and the Angevins: Province and Empire 1158-1203 (Cambridge University Press, 2000)

Michael Jones, The Creation of Brittany: A Late Medieval State (Continuum, 1988)

Patrick Galliou and Michael Jones, The Bretons (Oxford University Press, 1991)

André Chédeville and Noël-Yves Tonnerre, La Bretagne féodale: XIe-XIIIe siècle (Ouest-France, 1987)