changing encoding of summary buffer

[Sébastien Kirche]

Hi,

I am happy user of Gnus for years, but since I have switched to a system
with an utf-8 locale I have a problem with the dates displayed in the
summary buffer when month name contain some accents (French locales).
Example:
--8<---------------cut here---------------start------------->8---
R  |déc 15 '16| James K. Lowden         |    0|  3.7k | ┏● Ubuntu font of 
wisdom
R  |déc 16 '16| Javier                  |    0|  1.7k | ┗━━❯ 
O  |déc 17 '16| James K. Lowden         |    0|  3.5k |  ┗━━❯ 
--8<---------------cut here---------------end--------------->8---

Here 'déc' stands for 'déc' the abbreviation for 'décembre' (=
december).

I can see from the mode-line that the summary buffer is in latin-1 /
iso-8859-1 encoding (while my prefer-coding-system is utf-8), and did
not find if it can be changed to another one. 

I am with emacs 24.5.1 on Mac OS X, and my gnus 5emacs built-in) is
5.13. 

Thanks for any help!

FYI, my gnus-summary-line-format is defined as
--8<---------------cut here---------------start------------->8---
(setq gnus-summary-line-format 
      (concat
       "%*%5{%U%R%z%}"                          ;cursor pos / Status / Replied 
/ score
       "%4{|%}"                                         ;
       "%2{%-10&user-date;%}"           ;call gnus-user-date, according to 
gnus-user-date-format-alist
       "%4{|%}"
       "%2{ %}%(%-24,24f"                       ;Space + From: or To:
       "%4{|%}"
       "%2{%5i%}"                                       ;score
       "%4{|%}"
       "%2{%6k %}"                                      ;pretty-printed size
       "%4{|%}"
       "%2{ %}%3{%B%}%1{%s%}%)\n"))     ;space + tree + subject (if root, else 
empty)
--8<---------------cut here---------------end--------------->8---

and my gnus-user-date-format-alist
--8<---------------cut here---------------start------------->8---
;; affichage de la date en relatif
(setq gnus-user-date-format-alist
      '(((gnus-seconds-today) . "     %k:%M")                   ;dans la 
journée =      14:39
                ((+ 86400 (gnus-seconds-today)) . "hier %k:%M") ;hier           
 = hier 14:39
                ((+ 604800 (gnus-seconds-today)) . "%a  %k:%M") ;dans la 
semaine = sam  14:39
                ((gnus-seconds-month) . "%a  %d")                               
;ce mois         = sam  28
                ((gnus-seconds-year) . "%b %d")                                 
;durant l'année  = mai  28
                (t . "%b %d '%y")))                                             
                ;le reste        = mai  28 '05

--8<---------------cut here---------------end--------------->8---

-- 
Sébastien Kirche