Hi foid!
I set up a fresh Xfce environment for testing. I succeeded to build a French Xfce desktop.
What I did:
Add "fr_FR.UTF-8 UTF-8" to /etc/locale.gen
Execute locale-gen
Select fr_FR.UTF-8 with eselect locale set
Reboot
What I did not:
Set LINGUAS (As you may know, setting LINGUAS reduces language supports. Without LINGUAS, Xfce installs French language support by default on my environment)
Set L10N (Doing so should not harm but I think it is not necessary in this case. It is recommended to set if you install Firefox, for instance.)
Set LANG in xinitrc (as eselect locale updates /etc/profile so the setting should be enabled system-wide anyways)
Would you show us the output of the following command?
env | grep "^\(LANG\|LC_\)"
