Wow, talk about screwing up the meaning of a last name for someone. Diaz de Leon is in fact Spanish. However, Diaz de Leon originally means Diaz (son of Diego) and de Leon (of Leon, Spain).
Diaz de Leon is not French by any means. It is Spanish in whole. Diaz de Leon is Diaz (son of Diego) and de Leon (of Leon, Spain). We no longer consider 2 names. It is one, Diaz de Leon.
According to immigration and census records, Diaz De Leon is Spanish in origin, with the Leon part being added meaning "son of Leon". Leon in it's own right is French in origin. I will continue to try to find additional information and will let you know if I come across anything else.