gnus-group-posting-charset-alist

gnus-group-posting-charset-alist is a variable defined in `gnus-msg.el'.
Its value is

(("^\\(no\\|fr\\)\\.[^,]*\\(,[ 	\n]*\\(no\\|fr\\)\\.[^,]*\\)*$" iso-8859-1
  (iso-8859-1))
 ("^\\(fido7\\|relcom\\)\\.[^,]*\\(,[ 	\n]*\\(fido7\\|relcom\\)\\.[^,]*\\)*$" koi8-r
  (koi8-r))
 (message-this-is-mail nil nil)
 (message-this-is-news nil t))

Documentation:
Alist of regexps and permitted unencoded charsets for posting.
Each element of the alist has the form (TEST HEADER BODY-LIST), where
TEST is either a regular expression matching the newsgroup header or a
variable to query,
HEADER is the charset which may be left unencoded in the header (nil
means encode all charsets),
BODY-LIST is a list of charsets which may be encoded using 8bit
content-transfer encoding in the body, or one of the special values
nil (always encode using quoted-printable) or t (always use 8bit).

Note that any value other than nil for HEADER infringes some RFCs, so
use this option with care.

You can customize this variable.