In the area where I was born (between Antwerp and Mechelen), Frans happens to be one of the most current family names. It's also a first name. I know of at least two people who are named Frans Frans - how cruel parents can be!
I'm pretty sure that all you Frans-people originated here, in Flanders, Belgium.
My reasons for believing this ?
* It can't be a German name - then it would be Franz.
* The name is very rare in Holland.
* Frans in (Flemish=)Dutch also means: "French" - which could explain the original meaning of the name.