Three suggestions.
1) Grab the repo and redo it specifically for the Russian language. We have people who are able to do it. The problem is that it will be tedious to keep the localization up to date with the mainstream versions of Doom RL.
2) Try to translate the way where declension stops being an issue. The problem is that success of this plan is doubtable, will require a fair part of imagination al well.
3) Deal with it. This will work for sure, but the problem is that it sucks.
1) is definitely the way I would personally like to go. There will be quite an effort in the beginning - but I don't believe there will be too much text to change between the versions (and the devs will certainly let us know in the changelog).
And of course, we would have to be able to change the repo to each and every particular language. Given the fact that Czech is the language with second toughest grammar structure in the world (at least that's what I heard), this would really be Tough As Nails.
Last but not least - this would require some serious programming skills to see how the messages are constructed at first, then creating the grammar structure and then applying it into the game.
Last but very first - DoomRL is closed-source so far. I believe that a translation squad would be given access to some portion of needed source code - but ingame testing would probably be a completely different issue.
2) At some point better than at the others:
"Here lies: a medkit" and "The sergeant uses: a medkit" sounds certainly not that awkward. But let's say:
"You attached: a power mod to: a pistol" (not a real game example, but usable for this consideration) would be way tougher without going the 1) way.
3) Always an option. But the goal of attracting non-English-speaking players to DoomRL would then be rather screwed.
All in all, I can barely offer any other suggestion than to create a dictionary for each language, with all the declension needed, and a list of the "on the fly" generated messages. Most difficult to start, I believe, but also the easiest to keep up with further versions.