For now, see httpsTo do a test with dark lang use the update_lang url as shown here: http://edx-wiki.atlassian.net/wiki/display/ENG/Setting+Up+Sandbox+for+Localization+Test
Will port to here once the associated pages it links to are ported to the open instance.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