As far as I know (I am no fan of Sci-Fi, so cannot give you more from that genre) many books of Stanisław Lem are published in English translation (Solaris, Tales of Pirx the pilot, His master's voice, Eden, Peace on Earth etc.), published mostly in the 1980's and 1990's.
I think that it is not easy for a Polish writer to make appearance on the English-language Fantasy market, huge but crowded with Bards of the West ;(
I tried searching a few names in the Index Translationum, databases.unesco.org/xtrans/xtra-form.shtml but without a good result... (I did not search for Sapkowski, because you know him.)
Stefan Grabiński called "polish Poe" or "Polish Lovercraft", excellent but unfortunately undervalued Polish creator of horror fiction. I love his scary stories written in old-fashioned way set at the beginning of 20th century.
His novels: Salamandra (Salamander) (1924) Cień Bafometa (Shadow of Baphomet) (1926) Klasztor i morze (Cloister and Sea) (1928) Wyspa Itongo (Itongo Island) (1936) and collections: Na wgórzu róż (On Rose Hill) (1918), Demon ruchu (The Motion Demon) (1919), Szalony pątnik (Mad Pilgrim) (1920), Niesamowita opowieść (Incredible Story) (1922), Księga ognia (Book of Fire) (1922) Namiętność (Passion) (1930)
English webside about him: latarnia.com/stefangrabinski.html
Jan Potocki "The Manuscript Found in Saragossa"; one of the best Polish books I have ever read: "The sixty-six stories cover a wide range of themes, subjects and styles, including gothic horror, picaresque adventures and comic, erotic and moral tales. The stories reflect Potocki's interest in secret societies, the supernatural, and oriental cultures, and they are illustrated with his detailed observations of 18th-century European manners and customs, particularly those of upper-class Spanish society" - source: en.wikipedia.org/wiki/Jan_Potocki
Some books of Andrzej Sapkowski have English translations, being more precise there are 3 books with short stories, they are a prolog to the 5 book saga, these stories are called (after wikipedia) The Last Wish, A Sword of Destiny and The Time of Contempt.
As for the saga only the first tome was translated until now : "Blood of the Elves".
I can recommend Sapkowski's books, he's world is not as black and white as it is in Tolkien and many other writers and there's much more politics and scheming (not in these 3 first novels though). A great read.
