gnus-group-ignored-charsets-alist

gnus-group-ignored-charsets-alist is a variable defined in `gnus-sum.el'.
Its value is

(("alt\\.chinese\\.text" iso-8859-1))

Documentation:
Alist of regexps (to match group names) and charsets that should be ignored.
When these charsets are used in the "charset" parameter, the
default charset will be used instead.

You can customize this variable.