A piece of advice:
Do printenv in a terminal and let people know if
GDM_LANG
is set and to what value.
This depends on your choice in the gdm login screen for language.
Hopefully your trouble is just a choice of the C locale for language
(or most likely you never chose one) or never installed appropriate
locale support.
I cannot reproduce your problem. I have
GDM_LANG=pt_PT.ISO-8859-1
but I am an ultra conservative about using ISO-8859-1 which makes life
with pine, latex and php simpler.
Duplicating a file in nautilus I got a filename with "cópia" which Vi
improved happily opened.
On the face of it your problem is totally unrelated to gnome.