Brittany was and is inhabited by another people with a quite different culture and a different language from that of the Franks, the Breton language is Brythonic Celtic and has much in common with Cornish and Welsh. There are other Celtic languages known as Goidelic and this grouping includes Irish, Scottish and Manx.
In Roman times Brittany was known as Armorica and the Celtic Tribe who lived there the Venetii. The language spoken by the Venetii is unknown, as at the end of the Roman period, Brittany was flooded with Romano-Celtic refugees from the Anglo-Saxon invasion of mainland Britain.
Brittany was then itself invaded by Vikings as was normandy further east the vikings who invaded normandy became known as Normans and they invaded both England and Brittany. when Henry of Anjou successfully claimed the English crown, he made his son Geoffrey Duke of Brittany. both Geoffrey and his son Arthur died in mysterious circumstances.
As a result of the deaths of Geoffrey and Arthur the inheritance of the Duchy became to say the least, complicated. it was not made ant easier when John III died in 1341 without issue.
By 1355 there were two separate claimants to the duchy—Charles of Blois, by virtue of his marriage to Joan of Penthièvre and John of Montfort. John had a better claim and was supported by King Edward III of England, Charles was championed by the Franks.
Chateau Monmuran lay on the very edge of the zone of Frankish influence as did Broons, the birthplace of Bertrand du Guesclin