Well, it is true that if there is not much about certain royals in a language, like English less is known of them. I think the statement concerning English though had more to with the centrality of the English language, and how it is important to the world, and so on, and thus that makes the royals of that country particularly well known, if not important. That is true, although the observation about books in languages is true as well. If there is not something quite headline grabbing about these dynasties, and royals usually there isn't much written about them in English, no matter how much you would read about them if there was. I know there are royals that interst me, that I can't read about as much as I would like because there is just not enough about them in English, or at least books that I can find, like the Hapsburgs of Austria.
The royals of those Scandinavian countries are not dull, perhaps, but just not as interesting. If you know much about their history you realize that. Chrustina of Sweden was an exception, she has interested people more than most. But the rest of them have been largely forgotten, whether that is relevant or not, at least in the English language, although I can't speak for any other, myself. Less historically, and more recently, have not the royals of these countries been accused of popularizing monarchy, that is making it seem more common, and less regal and full of distant glamour? The Windsors have tried to keep the mystery, although there have been, in the past so many scandals that perhaps they don't have that anymore. It is said the royals of the Scandinavian countries are more about just acting like everybody else, and maybe that makes them seem less interesting, because there is more mystery in the Windsors, and other dynasties?