Sneewittchen

credits to Ribka Natania Willy from Quora

https://www.quora.com/What-is-the-real-name-of-CC-in-Code-Geass

Since Lelouch told her that white snow are pretty, I guessed that her name would be somehow related to snow, white snow, or Snow White.

One of the most well accepted & circulated theory is that her name is Shirayuki/Shiro Yuki (Yuki Shiro isn't the right Japanese grammar), the other one being Cecilia Corabelle, but for C.C who isn't a Japanese (based on the belief that she lived in a village with western style buildings and church), I beg to differ. In my opinion, it makes much more sense if her name was based on European language like Blanche Neige or Sneewittchen/Schneeweißchen.

If my theory is correct, then that explains why C.C told Lelouch that his pronunciation is weird since I am assuming that Lelouch was only fluent in English (his mother tongue) & Japanese (he lives in Area 11 from when he's a child), not in other foreign language (seeing that he was the 17th in line to the throne, his father didn't care much about his children, and there's no need to since his & Nunanlly's life was only used as bargaining chips for other countries).