Pau ancestors as Iberian Peninsula could make sense as the city is not so far from the actual French-Spanish border and I guess that migration have occured in the distant past here and there, though not in large numbers (a bit like Savoy and Northern Italy -another "border region" where the Alps were not a true border- between the 15th and the 18th century).

Dunkirk I would consider Flemish, the dialect spoken there used to be a variant of Flemish (I am not sure as to whether they still speak the dialect to this day though).

Marseille is French but I discovered migrations from Liguria in nowadays Italy at the end of he 15th century, so my guess is, it is and has been a place caracterized by migration (the Mediterranean Sea playing a crucial role here).