One thing most Eastern European countries shared, especially during Soviet times, is a wide-spread fascination with the West. In the absence of free access and real interactions with it, Eastern Europeans would often see it as a promised land, any connection with it treasured, any relative who lived in the West revered.
Even before the Iron Curtain made Western Europe hard to access, Eastern European aristocracy in 19th and early 20th centuries spoke French in social circles. If you wanted to appear sophisticated and make an impression, conversing in French might have done the trick.
Watch out for all the clues director Julie Bertuccelli sprinkles throughout the film to suggest how characters idealise France.