Jantine is Dutch and it would be the Dutch equivalent of the English Joannie.
Jan(s) is the Dutch form of
John and the female form is Jantje or Jantien/Jantine, meaning 'little
Jan'. Dutch female names are often male names with endings like
-(t)je or
ien/ine . The last form is also an ending in
France, e.g.
Jeannine,
Nadine,
Caroline, etc.