I came across a 1977 article on Mary Stewart in Something About the Author* that says that "Mary Stewart's novels have been translated into sixteen languages, including Hebrew and Icelandic and Slovak." I'm assuming her books have spread even further in the last 25 years, but I'm not sure how to find out the exact number. I cruised the web for cover images and was able to find The Crystal Cave in French, Spanish, and German. Very interesting to see the different cover art used in different countries. The cartoon cover on the Spanish edition is pretty cool.
The National Library of Scotland has an extensive collection of Mary Stewart novels, including many translations. If you search in their catalog, you'll come up with 458 hits for Stewart, Mary, 1916-. That's a lot of different editions. Kasteel in de Alpen is Nine Coaches Waiting in Dutch. Manadisirnar is The Moon-Spinners in Icelandic. Okami mori no noroi is A Walk in Wolf Wood in Japanese.
Of course, it's not at all surprising to me that Mary Stewart is read all over the world. Her protagonists may be perfectly English women, but a good story is a good story the world over.
*Something About the Author, v. 12: 217-219. Detroit, Mich.: Gale Research, 1977.