>One of the problems in language and especially accent learning is that we lose in childhood the ability to distinguish sounds that we do not hear.

That's right. My African foster son came at the age of five and in 4 to 5 months he could talk fluent with his class mates. Now 22 he speaks without a trace of accent. His sister who came in also at five with her Franco-African mother speaks with the accent of her mixed mostly immigrant school.A second sister who was born and raised here also speaks the immigrant schooling accent , because they never had the real native accent at home.
My son grew up only hearing the Dutch accent for his starting years.

I always admire people who can hear which part of a town or country someone comes from.

I remember from a period we often were in contact with Belgian friends one day someone came at the door and on hearing me asked: "are you from Belgium too?" Without noticing I had taken over the softer sound from our Flemish friends.