This is indeed a bit mysterious. So I've done some more research and Ifound out that St.Foster is the English name of St.Vedastus, also known as St.Vaast or Wadast (so from V to F it's not really a big step). And this guy was also called
Gaston. So the real question should be: How did Vedastus become
Gaston? This site lists two possibilities:
a) It was influenced by the German(ic) word "gast" (guest, stranger)
b) It has something to do with Gascony
Now, is this more satisfying?
Andy ;—)