you can download a russian translation here:
http://xenforo.com/community/resources/russian-language.2/
and install it as RU-Master. After that, you can create a child language:
RU-Master
- My custom russian language
where you make all your custom changes for add-ons etc.
In addition you can install this add-on:
http://www.xendach.de/resources/limit-language-usage.26/
To make your custom language the default language and disable the possibility of choosing another language for your users. Works great on my forum.