You can easily change language of UT3 Steam version in

My Games\Unreal Tournament 3\UTGame\Config\UTEngine.ini

in line:

Language=INT

(for example for me it works with POL - Polish)

Why can't we chose on Steam which version of the game we want to launch?

Red Orchestra is on UT2004 engine and it has an option to chose it's language from the Steam Properties menu.

I think it can be done and should be done in Unreal Tournament 3 on too!