... your mouse over each iLife application icon and control-klick it. Choose "Show package contents" and go to the folder "contents/resources". There you´ll find lot´s of foreign language folders. Just keep "english.lproj" (or the language you like the most) and trash the rest of those ".lproj" folders.
Actually doing this freed more than 700mb with keynote alone, as the language packages contained huge graphics template files.
By the way: Each update via Apple (and third parties) will bring all language files back and you´ll have to start it over again.