fix bug 647796 - get_best_language without NSL bug

https://bugzilla.mozilla.org/show_bug.cgi?id=647796

Same change as #918 but I made our NON_SUPPORTED_LANGUAGES empty for now. To test, to to https://developer-local.allizom.org/ with default language, supported language (e.g., French 'fr') and un-supported language (e.g., Serbian 'sr').