Hey, somebody!!! Misters developers, answer something on a theme, please.
Okay, I don't know all the answers but I'll tell you what I know. FS2 had localisations for Polish, French and German. As far as I know there never was an official localisation for russian. That means I have no clue if code changes would be needed in order to make it work.
What I do know is that FS2 works out which language should be used by looking at a registry value. If it finds a language it knows how to deal with it automatically translates the lines into the other language. In order to make a good Russian localisation you shouldn't edit the text in the mission files directly. Instead you should change the -1 at the end of each file into a number. You then edit the corresponding number in Strings.tbl.
In a similar fashion you'll also need to edit the TStrings.tbl file in order to update the interface.
All these files are in the main TBP or campaign VP file. You can get at them using VPView 1.3 (Check my FAQ's tool page)