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). The content of the Quick

Reference in the "Help" menu can also be translated, see 

https://bugzilla.gnome.org/show_bug.cgi?id=307856#c3 for a how-to.