Currently, the Gliffy Plugin for Confluence only operates correctly when character encoding are set to UTF-8 in the Administration General Configuration.
We know of at least one customer who was running ISO-8859-1, and ran into problems using Gliffy.
http://wiki.gliffy.com/display/proddev/Java+code+style+guide#Javacodestyleguide-Makesureyouknowwhatyourdoinginregardstocharacterencoding%2Fi18n