Evolution is translated like most GNOME projects, by updating the
.po files in evolution/po and by translating the help files in
evolution/help/C and placing them in a locale specific dir, ie
evolution/help/fr.  In addition, the default welcome message is in
evolution/mail/default/C/Inbox and can be translated by translating the
message in the file and also putting it in a locale specific dir, ie
evolution/mail/default/fr/Inbox.  The file name 'Inbox' should not be
changed, as this is an internally defined string (the name in the
folder list will still be translated though).