I don't think there is a solution, I am almost certain that the sound files for TATW have been compiled in English only. This means that additional sound files added by TATW will be in English but that any sound files that are just copied over from MED2 will be the language that you have that game in.
I don't think there is anything you can do except for possibly getting the base game in English. Surely this isn't a problem though because if you WANT English voices but can understand the other language perfectly then you can at least understand what everyone says?
