Strange charset selection when posting in iso-8859-1

[Peter Simons]

Hi,

I've run into a problem with Gnus 5.9.0 when posting articles (or
e-mails) that contain ISO-8859-1 characters. When I hit C-c C-c
to send the posting off, Emacs asks for the charset the article
would be using, but the selection it offers doesn't contain
ISO-8859-1! There is "latin-iso-8859-1" (or something like that),
but that's not a valid charset name for RFC articles, so I
essentially can't post non-ASCII articles at all right now. :-(

The problem started to appear after I added

  (setq-default enable-multibyte-characters nil)

to my ~/.emacs file. Now, I don't want multi-byte mode enabled,
but I don't want to forego umlauts or guillemots either.

Does anyone have a recommendation for me how to remedy this
problem?

Peter