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 (with ?preview-lang=using language eo) and RTL (with ?preview-lang=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