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The implication in the pilot is that Mary has lived in France since she was at least 6; she and the Dauphin both mention their times together at court before she was sent to live at a covent for a few years. When "Reign" opens she's returning to the court for the first time in several years.
So TPTB got that historical detail right (or close.)
(I think I got it right: the dialogue was hard to understand most of the time due to the horrid background msic and the captioning lagged a second or two behind the dialogue the entire hour.)