It's not 'Brittany, it's western Brittany, the land opposite Domnonea & Kernow, eastern Brittany has a different language called Gallo. The western part had a kingdom of Devon and another of Cornwall; Devon no longer exists but because Cornwall does, the Breton version is 'Great Cornwall', however, in French it's 'English Cornwall'. Brittany is situated on the Armorican peninsular which 'Armorica' is the historical name i.e. pre-Welsh & Cornish. The first Britons crossed the channel to protect the Roman Empire against the invading VisiGoths, through treachery they lost, the army was scattered but some made their way to Armorica, and according to Geoffrey of Monmouth they cut out the tongues  of the woman in order to preserve the purity of the language. The result of all this was that Britain was denuded of its army leaving itself wide open for attack from the Irish with no self-defence. This is where Vortigern comes in by asking for help from Hengist & Horsa, we all know where this led to don't we? Having the original settlement in place the Welsh and the Cornish knew where they could come for sanctuary. Most of the Breton 'Saints'/Archbishops were born in Wales and St. Illtud was born in Brittany. The yellow pestilence/plague of the 6th century also contributed, Breton St. Illtud came back to Brittany to stay with Welsh St. Samson of Dol, but it wasn't just Brittany there was also Bretania in Galicia, a Spanish region where they speak a dialect of Portuguese. Many Breton words were taken from south east Wales that have almost disappeared having been replaced by a standardised Welsh in schools using North-Walian words never before heard in the south, it's the same in Brittany they are now standardising their dialects into one language. By the way the onion sellers did brush up on their Cymraeg/Welsh. One can pick out many Welsh words but rarely the full sentence, also constant transpositions: eu = aw; th/dd = zh, mawrth = meurzh; w = ou, dwr =dour; y = e, mynydd; sul, lun, meurzh, merc'her, yaou, gwener, sadorn. Nuances: Awel/Avel is breeze not wind, Dorn is hand not fist, sgouarnog is ear; cadair = kador;  haul = heol; mynd maes (pronounced as in fairs) = mond er maze; coed = koad; troed = troad. Methinks I will leave you now before I over-over do it. P.S. Spit out the legend from the fact, it's a bit of a mixture. For reference see Gwyn Griffiths who worked for the BBC he had a great book on the relationship between the Welsh & Bretons that I lost when my pub went up in smoke, although he did promise me a replacement; he also wrote a book on the Johnnies.