Napoleon tried to regulate naming conventions by having a list drawn up of acceptable French names, based on the Bible, Greek and
Roman history, Catholic saints etc. I have a feeling that Breton names were excluded. I also recall that about 20 years ago at least, if one had a baby in
France that child had to have the French form of its name; so my son
Peter, born fortuitously in England, would have been
Pierre if he'd turned up in
France instead.
Whether this still obtains I don't know.