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 cannot 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.
To quickly decompile all LUC files, I have created a batch file containing the following commands - feel free to re-use it:
@echo off
for %%f in (*.luc) DO LUADEC.EXE %%f > %%f.out