Another thing to consider - although I have to admit not being certain on this one - is that Alix had also learned French and German, in addition to English, prior to taking on Russian. Grace Kelly I don't believe spoke anything but English before moving to Monaco. When one has already had to go through the process of learning multiple languages, and at a younger age to boot, does it not make it easier to learn more?
Firstly, I read somewhere that Grace's mother, the daughter of German immigrants from the upper class of Württemberg, tried to make her daughters learn German, but they protested, saying that after WWI German was an unpatriotic language for Americans. Any Grace Kelly fans out there who know or can check biographies?
Anyway the German element must have been substantial in her childhood, with her German maternal grandmother (who actually was Hessian by birth) living untill 1949 and dying in Philadelphia.
To actually grow up as bilingual, like Alexandra did, greatly facilitates language acquisition later in life. Just learning or being taught another language does not necessarily do so, it might just as well put the child or adolescent off all language learning in the future. (Especially if it's connected to a conflict with the parents, like it appears to have been in Grace's case and / or done with tedious methods of cramming without focus on actual communication.)
How much did Princess Grace struggle with French? Was she simply a slow learner in this particular area or was her French poor for the rest of her life?
Considering that one documentary said she worked on her Philly accent during her acting training, recorded it and listened to it and succeeded in transforming it into the legendary mid-Atlantic accent of old movies I don't think it was lack of ability or drive. Rather, there were probably psychological reasons in addition to lack of immersion into a Francophone environment.
Here is a clip from a documentary narrated by the wonderfully snooty, pedophile snob and former French minister of culture Frédéric Mitterand in which she speaks French:
http://www.youtube.com/watch?v=QmPgF1NXN5E Starting at 1:00, it sounds like she is searching for the words and not quite at ease, but it's fully comprehensible French. (You hear a lot of strange accents on French TV, from bilingual countries like Switzerland and Belgium, Quebéc and former colonies) just as much as on American TV, I presume.)
I read somewhere her French improved over the years and she practised by reading (and acting out?) French plays.
Grace, on a smaller scale, being from one on of the more renowned aristocratic families in America.
Are we being a bit too patriotic on behalf of a fellow Philadelphian? :-)
They were not an old family like the Bouviers and were not accepted by Philpadelphia Main Line society, I've read.
There are some very intelligent people who simply don't have a knack for languages.
There is the famous left and right brain divide, some being more into the communicative aspect, others more into the systematic aspect, but I think all intelligent people easily can learn a language if they have to and set their mind to it. But not all intelligent people see the need for it, some are so driven by a thirst for new knowledge that they find it unnecessary to learn new ways to say and hear the same things.
But from the top of my head I can't think of any very intelligent person who struggled to learn languages.