Can I set the dictionary based on the group I’m posting to?
Yes, say something like
(add-hook 'gnus-select-group-hook
(lambda ()
(cond
((string-match
"^de\\." (gnus-group-real-name gnus-newsgroup-name))
(ispell-change-dictionary "deutsch8"))
(t
(ispell-change-dictionary "english")))))
in ~/.gnus.el. Change "^de\\." and "deutsch8" to something that suits your needs.