Mplayer is not imported in Launchpad/Rosetta because it's a package from main and we are not (yet) able to handle those translations with language packs so what the package ships is exactly what upstream had at the time it was packaged in Ubuntu.
What I don't know, and should be checked by Mplayer maintainers in Ubuntu, is whether the package has any problem with the Russian translation provided by upstream and that's why you get it in English. You will need to wait for them to get an answer on it.
Mplayer is not imported in Launchpad/Rosetta because it's a package from main and we are not (yet) able to handle those translations with language packs so what the package ships is exactly what upstream had at the time it was packaged in Ubuntu.
What I don't know, and should be checked by Mplayer maintainers in Ubuntu, is whether the package has any problem with the Russian translation provided by upstream and that's why you get it in English. You will need to wait for them to get an answer on it.