Hi and Happy new year !

Following an idea from Alex Schroeder[1], I have hacked something that
could be of interest for people (I guess ;)). As I said in another
post, I am not familiar (yet) to gnus internals and so, my "patch" is
actually some elisp I wrote separately.

What it does is quite simple, it adds a new gnus-parameters variable
(dictionary) which allow to enable flyspell with per-group dictionary.

That way, one can simply use it is to say something like this:

,----
| (setq gnus-parameters 
|       '(("list\\..*april.*" 
|          (dictionary . "francais"))
|         ("list.debian-user-french" 
|          (dictionary . "francais"))))
| 
| 
| (setq gnus-parameter-dictionary-alist
|       '(("ml.emacs.ding" . "american")))
`----

And whenever he enters those groups, and reply or post something, it
will launch flyspell with the right dictionary.

Here us my add-on:

,---- 
| (gnus-define-group-parameter
|  dictionary
|  :function-document
|  "Return the dictionary used for the current group (if any)."
|  :variable-document
|  "Alist of regexps (to match group names) and default dictionary to use when composing a new message."
|  :parameter-type '(string :tag "Dictionary")
|  :parameter-document "\
| The default dictionary to use in the group.")
| 
| 
| (gnus-setup-dictionary "list.debian-user-french")
| 
| (defun gnus-setup-dictionary (group)
|   "Decide wich dictionary to use for group `group`."
|   (let ((group (or group ""))
|         dictionary)
| 
|     (setq dictionary (gnus-parameter-dictionary group))
|     (message "Dictionary used: %s" dictionary)
| 
|     (when dictionary
|       (set (make-local-variable 'ispell-local-dictionary) dictionary))
|     ))
| 
| 
| (add-hook 'gnus-message-setup-hook
|           (lambda ()
|             (gnus-setup-dictionary 
|              (if (boundp 'gnus-newsgroup-name)
|                  gnus-newsgroup-name ""))))
`----

IMO, this code can be enhanced but it works already as-is so feel free
to use/copy/paste/modify/incorporate it in gnus. My work is 
licensed under the GNU General Public License and I also let my
copyrights go to the FSF (if needed).

Cheers,

zeDek

Footnotes: 
[1] http://www.emacswiki.org/cgi-bin/wiki.pl/GnusSpelling
-- 
Xavier MAILLARD (GnuPG: 1024D/1E028EA5)
EmacsOS user (http://emacsfr.org)
APRIL (http://www.april.org)