As far as I know, the only way to do this is to mod the game files themselves. I'm not sure how to do it, but if you copy the xml files for the civ you want to rename, make your changes and put it back it should work. Just remember that it's not as simple as merely changing one civ's name. You'll start getting errors and crashes if you mod the in-game stuff but not the things that reference whe values you change.