What is the translation for 'une coutume bretonne' please

Bonjour Andrew, This one translates as «a custom from Breton». So here is an excerpt from Wikipedia referencing «Breton(s)»: The Bretons are an ethnic group located in the region of Brittany in France. They trace much of their heritage to groups of Brittonic speakers who emigrated from southwestern Great Britain including Cornwall to avoid the invading Germanic peoples. They also descend from the Gauls and in some parts from Vikings. They migrated in waves from the third to ninth century (most heavily from 450-600) into Armorica, which was subsequently named Brittany after them. They also have certain customs and dress that is unique to their culture. J'espère que cela vous aiderait.