I had a friend ask me this the other day. He had aspell installed and it had a Canadian dictionary, but setting the language to Canadian in his Emacs config file resulted in an error. This happens because the ispell module does not contain Canadian as a valid language.
The solution is to add Canadian to the ispell-local-dictionary-alist variable:
(defvar ispell-local-dictionary-alist
'(("canadian"
"[A-Za-z]" "[^A-Za-z]" "[']" nil ("-B") nil iso-8859-1)
)
)