I think you can change the language as long as you have a client below 1.27b (and have a cd key). Check this thread out. Change WC3 Language installation - #3 by multijet-2548
Unfortunately I don’t think you can use this method on any builds above 1.27b as this method requires you to reinstall the game. All versions need to be downloaded as a complete build due to not being able to patch past this point.