Hmm,
was this my suggestion?
i would try to keep this as simple as possible to avoid confusions.
a) don't expire the cookie
b) the cookies takes alway priority over database.
c) if the user changes her locale, the cookie is changed as well to the new value.
d) when the cookie is set, acs-lang (reached via pvt/home) should say, that the current locale is set via cookie, and give an option to clear the cookie.
