Breton is not a Germanic language but it tells us something about ethnogenesis in the migration age. A primary source shows there were four languages in Armorica: Roman (Latin-speaking Celt), British, Frankish, and a fourth language that must have been Alanic. Where does this leave Continental Celtic? There is, books don't tell you, evidence Gaulish was spoken in the Alps as late as the 12th century: but the Alps are far from Bretagne and mountainous regions were conservative (think of the survival of the Vasconic phylum in the Pyrenees but not in lowland Aquitane). François Falc'hun is confident that Gaulish did not die out before the Dumnonians arrived, mostly on the basis of Sidonius Apollinaris: but Caesar had denied the Gauls were even present in Bretagne during the Gallic Wars.

The fate of Gaulish is one of those language mysteries. Latin-Greek-Gaulish inscriptions from the 3rd and early 4th century, that Gaulish and Latin adapted locally to each other. The fate of Gaulish is a mystery but the racial stock of the Bretons is not: the physical anthropology shows the peninsula was a refuge for "Romans" during the migrations with little kinship to the Cornish Celts. Were Gaulish still in existence throughout Gaul it would surely have been brought to Bretagne independantly of Latin. But as we know it wasn't.