~ Her real name is Cecilla, not Ceceniah as she's made people believe.
~ Her historical title during Britannian-French War was "the Witch of Britannia", constracting with Jeanne d'Arc's title "the Witch of Orleans".
~ Assuming she was born in the British Isles or at least the young Britannian Empire, she was spoke English from a young age.
~ Then due to her own geass ("Absolute Affection"), many nobles had fallen in love with her and taught her French as well.
~ Since she's spent some time in the Chinese Federation due to the Geass Directorate being there and Mao, she's able to speak Mandarin Chinese.
~ And after helping Area 11 in the Black Rebellion, she should be able to speak Japanese rather well to get around.