We're so familiar with the tropes and concepts of US-centric SF, but imagine a whole new world of science fictional ideas - separated only by language! That's Soviet SF...
The finest series of translations was Macmillan's Best of Soviet Science Fiction in the 1980s. Some of the books are now very rare and expensive, but if you're a seeker of ideas, they're priceless (especially any stories by Genrikh Altov).
As an introduction, I particularly recommend World's Spring, edited by Vladimir Gakov. And one of the finest SF novels I've read is Self Discovery, by Vladimir Savchenko (freely available online here: http://lib.ru/RUFANT/SAWCHENKO/savchenko_selfdiscovery_ok-en...).
Also German, and 50 years old this year (!): the Perry Rhodan series.[0] It was being translated into English but I'm not sure if it still is. The earlier 1960s American English publications were very popular, I believe.
The Carpet Makers by Andreas Eschbach
Both are excellent.