Replacing region-locked games with global on Steam

A few months back I bought the "Polish-only" version of Black Ops 2 and replaced the Polish language files with the English manually. Because I'm tired of doing this, I wondered if I would be able to buy a new global BO2 key from a third-party retailer and activate the key on Steam "on top" of my original game, gaining access to download the English language files?

Solution 1:

You can now remove games from your account with Steam Help, please see this answer by r3tr0 t3h PeNgU1N oF d00m if you aren't sure how to do it.

You can use that to remove the old polish game and activate the global key.