How to make Quick Look preview text files in UTF-8 by default?
It's a file attribute problem. See here, at the end, two links explaining it: http://discussions.apple.com/thread.jspa?threadID=1479441&tstart=120
Basically, if you execute this: xattr -w com.apple.TextEncoding 'UTF-8;134217984' yourfile.txt your file will appear correctly in QuickLook.
In my opinion it has to do more with the editor and the way it saves the file than with QuickLook.