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.