Don't attach the locale filter until plugins_loaded

If for some reason get_locale() is called too early our filter function
may call wp_get_referer(), which isn't supposed to be called early and
we will fatal out.

See http://core.trac.wordpress.org/ticket/25294#comment:17


git-svn-id: https://plugins.svn.wordpress.org/admin-in-english/trunk@771025 b8457f37-d9ea-0310-8a92-e5e31aec5664