Files with .LUC extension are compiled LUA scripts. see LUA script in LUA format. When LUA files are compiled using a LUA compiler, they become binary files — these binary files are the LUC files you see in The Witcher. LUC files can be decompiled. I have used LUADEC 0.6 to do the job and I managed to decompile the majority of the LUC files. However, it seems that LUADEC 0.6 can not handle some of the LUC files in The Witcher: sometimes it gives an error, sometimes it totally crashes. Looks like an unfinished project — there is a bug report in their tracker about this.