One thing worth trying:
Change the left pane to Outline view and select everything there. Now change the laguage in the REVIEW tab. This will only affect things you could select on each slide with CTRL A (*ie not tables, smart art, non placeholder textboxes) but might get you closer.
We do have a
commercial solution which can change everything to any language. I'm afraid it isn't free though. The
free code on that page will change non placeholder textboxes and do a better job than the above. It will not change tables etc. The AddIn will of course fix everything in 2010.