At major international SF conventions, almost all the books on sale tend be in English. And yet, obviously, not all books are written in English.
An article at the Concatenation website analyses why so little is translated and what the "classics" in other countries are.
I can only speak to the works listed for Spain, and I think the list includes the important books by the most important authors. In fact, Worlds into Eternity
by Javier Redal and Juan Miguel Aguilera may be the best space opera I have ever read.http://www.concatenation.org/europe/european_sf_classics.html
— Sue Burke