Having ru_RU.KOI8-R locale I can see, copy and paste russian characters within the mrxvt. However, pasting from or to an external application, such as Firefox, Thunderbird, or gedit, makes text unreadable. If pasting from any app to mrxvt I have everything converted into '?' chars. Pasting from mrxvt changes russian characters into accented Europian characters. All other applications works correctly, so that must be an mrxvt problem.
Currently I'm using 0.5.2.