On overdue notice, subject is improperly displayed if it contains other characters than raw ASCII. In French, accented characters aren't displayed: àéèù, etc.
http://lists.koha-community.org/pipermail/koha-patches/2010-October/012682.html
Hi Frederic, I think this is a general problem for other email notices as well, see bug 4420. I ran into this problem while testing checkin notifications by email, I am not sure about other notification mails.
Patch pushed for inclusion in 3.2. Please test and close.