Fixes #1594

Fixed #1594 

Replacement pull-request for #1641

Added get_current_language() method and using it instead of translation.get_language(), also added get_language_code() method to check if language code is in LANGUAGES

+ tests for both methods

Common case is when in settings LANGUAGE_CODE = 'en-us' and in LANGUAGES it's 'en' which causes manage.py cms check to fail.