Well... The difference about understanding a language and actually speaking that language is quite big, at least that is what I experienced. My dad is orginally from Portugal but my mum is German, she doesn't understand a lot of Portuguese (she never learned the language) so my dad just talked to us in Portuguese when we were on our own without my mum. Usually we used to answer him in German, for some reason he never asked us to answer in the language he was talking to us. So today, when I go on holiday to Portugal I understand almost everything they are saying but I simply can't really answer
So it is very much possible that Marie-Chantal is understanding Greek but she can't answer how she wants to and so answers in English
Btw, does someone knows which language Constantine and Anne Marie are using to talk to each other and especially when talking with their children?