Strong support. Every single encyclopedia I checked, lists this guy's name as Casimir, with only minor variations after that.  Sokol's Polish Biographical Dictionary: Casimir III (The Great).  1979 Encyclopedia Britannica: Casimir III the Great, of Poland, Online Britannica: Casimir III , Webster's Desk Encyclopedia: Casimir III aka Casimir the Great. Columbia Online: Casimir III . This article should never have been moved to a Polish spelling.