Tinymce4 i18n

Language files have changed in tinymce 4.x. They aren't always based on a 2 characters language code (en.js, ja.js, etc.) anymore. This PR updates the twig extension to take that fact into account -- the existing code is broken for languages like hu_HU or ka_GE -- and to allow a reasonable matching between the available tinymce languages and the application locale.