Why none in Roman North Africa or all the other provinces in the east which were also "the middle of nowhere" with the same needs of metal work as Gallia/Britain.
Man of those countries speak Germanic languages now since they were invaded by Germanic speaking people after the Western Roman Empire collapsed, but there's still Roman ruins and old Roman cities to be found in all of those countries.
It depends.
We really have no data at all to back such a claim.
We barely even know anything about how metalworkers and other skilled craftsmen organized themselves back in those days in any part of the empire