The book A Dog of Flanders is popular in Japan and known as a very sad story. However, I heard that this story is about a loser and isn't very popular in Western countries. Is this true that it's less popular in Western countries?
This answer does not attempt to judge the book's popularity in Western countries in general, but in Flanders, where the story is set, and where I come from. (Antwerp is in Flanders.)
The novel was published in English in 1872 by Marie Louise de la Ramée (using the pseudonym Ouida). I never heard of her while studying English literature at university. The first time I heard about the story was through the comic strip adaptation in the Belgian/Flemish series Suske en Wiske, in an album named Het dreigende dinges (Dutch for "the threatening thing"; Willy Vandersteen, who invented the series, liked alliterations in titles). It was only after the publication of this comic strip that A Dog of Flanders was translated into Dutch (in 1987). ("Suske en Wiske" is one of the most popular comic strip series in Flanders; you could not grow up in Flanders after the 1950s without knowing Suske en Wiske.)
After this, you could still come across articles such as "Vlaanderen en Antwerpen ontdekken na meer dan honderd jaar hun geheime ambassadeur in Japan en Korea : een hond van Vlaanderen" (sorry, just a library catalogue entry). The title means "After more than 100 years Flanders and Antwerp discover their secret ambassador in Japan and Korea: a dog of Flanders"; the title implicitly suggests the relative obscurity of the story in Flanders in the early 1990s. I think the story is still quite obscure for people who haven't read the Suske en Wiske comic.
Update: There is a French translation of the book (possibly self published by the translator) but I have not been able to find a German or a Spanish translation.