I like Zimmerwald's idea about learning from or bringing along an NPC, assuming that the PCs realize that the servants know Russian. (While it says that on the NPC template, it's not something that would be obvious unless one of those NPCs were to tag along with the group.)
As for learning Russian through gaining skill points, I'm not sure how to handle it as far as the exposure necessary to learn. I'm not sure I'd let the PCs just 'learn' it because the Mantle gave them extra languages until they'd come to Earth, since even the concept of needing Russian isn't introduced until then, unless I'm forgetting examples of Russian inside the Hut, or something like that.
It's one of those things that I'd limit, much like Triaxian Common, unless the PCs came across a source that could TEACH it, since it's nearly non-existant on Golarion. If it were even regionally available on Golarion, I'd say that a PC could take it, but it's clearly not. Only the Baba Yaga and her progeny would have access to it on Golarion, and they wouldn't teach it, IMO.
It's the accessibility of the language that concerns me. That and my players tend to look at effectiveness of skills and from a statistical side rather than an RP side. I'm going to have to remember to play up Knowledge, Diplomacy and Linguistics for this one, since they often get pushed to the side since they're "RP" rather than "action".
Thanks for the additional ideas.
My 2 yen,
Akiosama