Yes, Factor K is stand-alone. I've played through it a couple of years ago and don't remember any game-breaking bugs. It's story takes place after the original TW after the Earth-Concordia war, main hero continues his work for secret service during the conflict with Yagnu aliens, if i'm not mistaken, encountering gigantic space installations on the way. Of course, all the text including story is in Russian, but me and other Russian-speaking members may help to describe what's going on before each mission. Flight interface is the same as in TW, though you'll need to perform a water landing, for example, and engage LARGE ground targets using cluster bombs. The game itself is shorter than TW, since it's a stand-alone addon.
If you are serious about buying Russian version of Factor K online, post here. I'll see what can be done, maybe make the tutorial describing what to click in the store and where. I don't think its wise to mess with your credit card stuff in a language you don't understand, so your best bet may be using WebMoney service to pay. Visit its English
site to decide whether you are willing to use their services to pay at 1C online (unfortunately, other payment methods are credit card or Russia-only). Overall, some English-speaking fans bought the Russian RigNRoll this way and play it right now (i don't know which payment option they chose though).