In all the years of collecting The Happy Hollisters, I had never seen a foreign edition until I saw the Spanish version of The Secret Fort, or "El Secreto Del Fuerte."
Pictured in these scans are two different types of Spanish Happy Hollisters books and a set of French Hollisters. In both sets, you can see the artwork is nothing similar to the US edition, in fact the Spanish and French books pictured here are of different artwork altogether.
The French books, at the top, are of the Sea Turtle Mystery (l) and on a River Trip (r). They are very unique in size, about 5x7 inches across, and slick picture cover editions. They have black and white and full color artwork inside. Below them are two Spanish versions of at Circus Island (l) and Punch and Judy Mystery (r). They are titled simply at the Circus and in Italy.
The white volumes below them are Spanish, too, but you can see of different artwork, although published by the same company, Ediciones Toray, S.A. The stories pictured in the white volumes are from the books, Mexican Idol, Pony Hill Farm and Ice Carnival respectively.
Some time ago I came across a British Happy Hollisters (through a bookdealer from Australia) and several Norwegian Hollisters. The British editions, published by World Distributors, has exact artwork as the US books, but the size of the books are roughly one-third smaller. They come with dust jackets where as the Spanish book and the Norwegian books are picture cover versions similar in style to the 1950 Whitman picture cover books. The British editions have illustrated endpapers similar to the US books, but the Spanish and Norwegian books have blank endpapers.
The British editions, which are not pictured, contain the same interior artwork as the US editions. The Spanish books have lots of artwork. The Norwegian books contain artwork exactly like the US books, but not every illustration is used. I counted only six illustrations in the Totem Faces book from Norway. The US books have a full-page illustration in every chapter. I will be adding pictures of the Norwegian books in the near future.
I would welcome input from other collectors who might know of other foreign editions of The Happy Hollisters.