WI: Breton ruled Neustria

How could we make Neustria ruled by Bretons, I wonder if a Vouille defeat can make this a reality with the Franks leader being defeated by the Visigoths and the Frankish leadership and Empire shatter which would allow the Bretons to rule Neustria.

If Bretons rule Neustria would the Bretons impose their language to the Gallo Roman speaking peasants in Neustria and would Breton be the dominant language of Neustria instead of the Oil dialects.
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