How does the Borderlands 2 Season Pass Region Lock Work?

I currently have a Russian key of borderlands 2 activated, which places both a RU copy and an EU copy in my steam library.  If i purchase the borderlands 2 season pass (ru), will it work for the borderlands 2 EU?

Yes, it will work perfectly. as long as the key you activated is russian, the season pass will work perfectly with the EU copy that is in your steam library.