Hello, I have a problem with localization of the game menu into Russian. when I try to enter Russian characters, squares appear instead of letters. how to fix it?

Menus are created using Adobe Flash and Scaleform. The symbols have to be embedded into the flash file for them to appear in the menu. You can do this from flash itself. Have a look at this https://youtu.be/krBrAGALIrU to see how text is created in Flash and font embedded.

You might also want to look into the localization process using the g_language. Look into the GameSDK file and localization, and you'll see english/german paks available. Make one for russian following the same rules and see if that fixes it for you.