have installed russian language pack but when i change administration(backend) language to russian K2 is still in english. Of course i had installed joomla russian language pack.
Any ideas?

Did you install K2's language pack as well?
getk2.org/extend/translations

Of course i did. And it is enabled. I don't understand why it doesn't work :(

Unzip the file you downloaded and see it indeed contains the language files. Sometimes Transifex might have some outages (rarely but it can happen).