For nowTo do a test with dark lang, see httpshttp://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 testing coverage (with ?preview-lang=eo) and RTL (with ?preview-lang=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