Is there any documentation how to use this? I searched already, but without success.
This is managed entirely through Launchpad/Ubuntu. Nothing has been contributed back to upstream, and nobody has volunteered to do the work of pulling those files into our source tree and maintaining them as they are updated (there is a lot of churn in translations; keeping them in synch with release code is quite labor intensive).
I would assume you can drop the .po file(s) into mailman/src/mailman/messages/ru/LC_MESSAGE/ and the build and install process will do the right thing.