Apparently the title has been in the works for a while, and an ongoing open beta phase is currently available in Russia. VG24/7 reports that there is a multi-language pack in the works, but until it's ready you'll need to bone up on your language skills if you want to participate.
For the uninitiated, S.T.A.L.K.E.R. is a series of post-apocalyptic single-player titles that mix first-person shooter action with sandbox RPG elements. Head past the break to check out the gameplay footage.