See ticket 14170. The global state _accepted is now reset when LANGUAGES setting has been changed.
I needed to change static.py to work around circular import issues.
Fixed #14170 -- Reset i18n cache when settings change
I have rebased the patch and added a small note about the added unit test.
Note that this is adding a new dependency between django runtime and the test framework (see https://code.djangoproject.com/ticket/20349)