Although this thread is pretty old, Microsoft has provided no solution that I knowwant to change the proofing language of. I suggest another approach for Mac users: open your presentation all my slides in Keynote, changea Powerpoint. I've tried setting the spelling language and possibly edit your whole presentationvia the Language Preferences menu, then exporthowever this only changes it to PowerPointfor new powerpoints.
Mac users have the option of opening the presentation in Keynote, change the language as required, and then export it back to Powerpoint