I know there’s a vast number of books published every year in languages other than English. I’m sure many of the more popular ones get translated into English. Are there many examples of books popular in their original language that have never been translated into English? If not bestselling then perhaps critically acclaimed?
Andreas Eschbach may be one of the best science fiction writers working today, if his first novel, The Carpet Makers is any indication. But it’s the only one of his books to be translated into English so far.
His work is critically acclaimed and he has had two best sellers in Germany, but *The Carpet Makers is the only book of his to be translated into English, and didn’t seem to sell all that well, despite acclaim over here.
Kurd Lasswitz was a MAJOR German science fiction writer and a contemporary of Wells. His “Auf Zwei Planete” , published at the same time as “The War of the Worlds”, also deals with Mars-Earth interactions and war. His martians are more humanoid and more advanced. They win, but establish a benevolent stewardship of earth. Exttremely interesting, and reads like an episode of Star Trek – for an early SF book, it has pretty advanced ideas. He was a MAJOR influence on Werner von Braun, Willy Ley, and that bunch in the German Rocket society and Peenedmunde. They named rocket parts after the names given them in his book. He also influenced Fritz Lang.
An incomplete translation of Auf Zwei Planete has appeared in English (and only once in paperback). Willy Ley translated two of his short stories into English. They were printed once, back in the 1950s in F&SF, and not reprinted. None of his other works have been translated, and AZP has never been fully translated.
Other German SF writers of the same period haven’t been translated into English at all.
Some of Stanislas Lem’s works still haven’t been translated from Polish to English. In particular, the only English translation of the twice-filmed Solaris is an indirect and incomplete translation made from a Polish-to-French translation.
Heck, there are still one or two Jules Verne novels that never got translated to English.
Among German authors, Karl May would be the most obvious example - the all-time bestselling author in the German language, but English translations seem to be few (list of titles compiled from the Amazon databases - as you can see all but for a few are in German.)
As an example of a contemporary bestselling author, most of Frank Schätzing’s books don’t seem to have been translated to English - I could only find translations of Der Schwarm (The Swarm, topped German bestseller lists for months) and Tod und Teufel (Death and the Devil - curious that this has been translated as it’s a medieval murder mystery with a lot of Cologne local colour). An example of a book that seems to be not translated yet is Lautlos. I wonder why - a thriller about a plot for offing Bill Clinton in a particularly inventive way should have been popular with a large readership in the US, shouldn’t it 
?
5 of his works are translated into French. You’re right that it is hard to understand why they haven’t been translated into English yet.