What is Sophie in French?

It is spelled as you have spelled it. Sophie is a proper name and not a nickname, as it is in English, for Sophia. Sophia was the goddess of wisdom in Greek mythology- there really never was a St. Sophia, it is an adjectival phrase the ( Holy Wisdom) and a personification of the idea- like Grace or Charity, both acceptable Christian names. so properly it is the former Chruch of the Holy Wisdom in Istanbul- as there never really was a St.Sophia unless some Byzantine empress or Greek Queen was canonized ( declared of sainted status) so it goes.