To be sure I understand what happens
You have an existing post in Russian with a tag named "tag". You edit the post content, don't touch the tags and you click on the update button and it creates a new tag named "tag-ru". Am I right?
Does it happen whatever you post is translated in another language or not?
Is synchronization for taxonomies activated in Polylang settings?
Did you try to check if this happens when no other plugin is activated and when you are using a default theme (tewenty twelve)?
