It does happen with an other user. I created a new user for the test, and the menu of the compose window present the same strange behaviour. The only plugin I think I'm using is the plugin to use bogofilter as spam filter instead of spamassassin, but I had not activated for my test with a new user (so the problem probably don't come from the plugin).

Concerning the requested commands, here is the result :

$ grep '_Éditer' /usr/share/locale-langpack/ -r
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/dia.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gimp20.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/epiphany.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/nautilus.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gimp20-libgimp.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gtkhtml-3.8.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/pan.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/contact-lookup-applet.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-cups-manager.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnumeric.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/xfdesktop.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/xchat-gnome.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-terminal.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gthumb.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/planner.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-pilot.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/rhythmbox.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gconf-editor.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/libgnomedb-2.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-volume-manager.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/yelp.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-media-2.0.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-session-2.0.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gnome-panel-2.0.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/gimp20-std-plug-ins.mo concorde
Fichier binaire /usr/share/locale-langpack/fr/LC_MESSAGES/evolution-data-server-1.8.mo concorde

$ strace -e open evolution 2>&1 | grep evolution-2.8.mo
open("/usr/share/locale/fr_FR/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/fr/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/fr_FR/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/fr/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = 10
open("/usr/share/locale/en_GB/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en_GB/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en/LC_MESSAGES/evolution-2.8.mo", O_RDONLY) = -1 ENOENT (No such file or directory)