Did Alex work more than Mary does now? I always hear she was very good at her role and just wonder what she did that Mary doesn't - other than being more selfless.
I admire how well they all still seem to get on.
IIRC, picked up Danish quickly and spoke fluently, which was difficult for Mary.
I seem to remember that Alexandra's skill in German was credited as a big help in picking up Danish quickly. Already knowing a language to a respectable level certainly aids in learning another and both German and Danish are in the Germanic Language family...German being in the West Germanic subfamily and Danish in the North Germanic subfamily.
I don't believe that Mary spoke a second language prior to learning Danish and then there's the thought that she and Fred spoke/speak English to each other which would make learning any second language more difficult as an adult. As an adult you must be motivated and really work at it to learn a second language.
Isn't English also a Germanic language? English is a West Germanic language that was first spoken in early medieval England and eventually became a global lingua franca. The Germanic languages are a branch of the Indo-European language family spoken natively by a population of about 515 million people. You had West- North- and East (extinct) Germanic languages.
The West Germanic languages include the three most widely spoken Germanic languages: English with around 360–400 million native speakers; German, with over 100 million native speakers; and Dutch, with 24 million native speakers. Other West Germanic languages include Afrikaans, an offshoot of Dutch, with over 7.1 million native speakers; Low German, considered a separate collection of unstandardized dialects, with roughly 0.3 million native speakers and probably 6.7–10 million people who can understand it (at least 5 million in Germany and 1.7 million in the Netherlands); Yiddish, once used by approximately 13 million Jews in pre-World War II Europe, and Scots, both with 1.5 million native speakers; Limburgish varieties with roughly 1.3 million speakers along the Dutch–Belgian–German border; and the Frisian languages with over 0.5 million native speakers in the Netherlands and Germany.
But you are, IMO, correct about learning a foreign language as an adult. It is (much) harder then as a child. And off course there are also differences in the natural ability of people to learn a foreign language.
As child I was relatively good in learning English and German, besides my native Dutch, and I credited it to my knowledge of and experience in Dutch and some variants of Low Saxon (I grew up with both). But others in similar circumstances had to put in much more efforts to learn the foreign languages.
BTW the French lessons I took at school were the 1st subject I got rid of
I had a hard time with this language.