The picture kind of says it all!
In the 1920’s and ’30’s there was a wave of books about Scandinavian, Slavic, Baltic, and other European cultures (basically, northern, central/eastern, and southern Europe), featuring lots of traditional dress and folk art.
Heavily dominating the field were Dutch books (represented here by The Dutch Twins); but there was also this wider phenomenon. In addition to the books pictured above, there was The Trumpeter of Krakow; Children of the Mountain Eagle; Vaino: A Boy of New Finland; Mountains are Free; and the English translation of The Wonderful Adventures of Nils.
The trend originated in the 1910’s with some fairly instructional “children of other lands” series’, including the Twins books and the Our Little Cousin books. These arose in the wake of waves of immigration from various parts of Europe to the U.S., presenting teachers and librarians with a rather different mix of kids than they were used to. Eventually such books broke out of their stodgy, educational niche and took on a certain literary cachet. Several of the books pictured up top are Newbery winners or runners-up.
I’ve spotted two later outgrowths of the trend. The first are books of the 1940’s and ’50’s about American children of different heritages or from different regional cultures, particularly the books of Marguerite de Angeli and Lois Lenski.
The second might be called the Scandinavian Invasion: translations of northern European authors into English starting in the 1950’s. This of course includes Pippi Longstocking, but also others such as the Moomin books and the Mrs. Pepperpot books. (That’s not the first Mrs. Pepperpot book, but it was the only cover I could find with the original illustrator.)
It’s also not an accident that those are both Puffin books (a U.K. imprint of Penguin). Many of these books first gained popularity in the U.K. and Commonwealth countries, and only much later made inroads in the U.S. I first met the Moomins and Mrs. Pepperpot in Canada during our year there when I was seven.