API i18n::$common_languages and i18n::$common_locales converted to Config API

They are now accessed via the Config API, and contain associative rather than indexed arrays.
Before: `array('de_DE' => array('German', 'Deutsch'))`, after: `array('de_DE' => array('name' => 'German', 'native' => 'Deutsch'))`.

Also fixed a i18n.js_i18n config accessor