Evan,
in DNN 4, go to Languages in Host menu and click "add language". Select "English (UK)" and save.
Go to Site settings and select "English (United Kingdom)" as default language, same for user profiles.
Go to Languages in Admin menu and disable "English (United States)".
in DNN 5.1+ go to Languages in Admin menu.
At the far bottom (or from module action menu) select "Add new Language".
For Language, select "English (United Kingdom)", for Fallback Language, select "English (United States)" and check "Enabled".
Go to Site settings and select "English (United Kingdom)" as default language, same for user profiles.
Go to Languages in Admin menu and select "English (United States)" in the drop down list.
Click "edit" underneath and on the new page, uncheck "enabled".
PS: Don't ask me, why the procedure became that difficult in DNN 5