To do a test with dark lang, see http://edx.readthedocs.org/projects/edx-developer-guide/en/latest/internationalization/i18n.html#coverage-testing
This is sufficient for both testing coverage (using language eo) and RTL (using rtl)
if you need to release languages, for example to test browser recognition when users are not logged in, see https://github.com/edx/edx-platform/wiki/Internationalization-and-localization#releasing-a-language