maksim.danilau: Hi, folks! There is a Russian community of sunless sea fans. And this community has localization created by them.
Also, there is a forum topic about localization in steam.
Is it possible to integrate this localization in beta branch or smth like this on gog?
You'd have to ask GOG directly for that (find an admin maybe?); however I don't think that GOG generally supports this kind of thing. One of the few exceptions is the series of fan-made patches for Vampire: The Masquerade: Bloodlines - it's not a translation but the fan-made patch is included in the download, primarily because without it the game is a buggy mess that's barely playable.
I think the best realistic outcome (unless GOG agrees to a separate branch or something) is to stick the localisation patch on a mod site (Nexus, ModDB, etc.) and then create a thread here telling people about it (and then get GOG to pin the thread), and from then on use the thread for updates etc. as required.
The other way to do it is to contact Failbetter and see if they're willing to support it officially - at the least they could endorse it (possibly providing a direct link to the localisation mod from within the game itself), or if they're really keen possibly even provide a language option in the game. I doubt the latter would gain traction, given the size of the studio and the fact that they're now working on Sunless Skies (I imagine doing that would be too much effort compared to just offering a link). It might still be worth contacting them though - after all, this could lead to extra sales for an entire language for little to no effort from them...