Trying to access Horizon using an Accept-Language header equals to "pt" (portuguese) or "it" (italian) returns an HTTP 500 making it impossible to access the dashboard. I didn't check other languages but I assume that it is not an italian or portuguese exclusive problem.
Changing browser configuration to use "en", "es", "fr" or "ge" works well and I'm able to access the dashboard again. But using italian or portuguese as primary choice for language, even having "good" languages in the Accept-Language, also makes Horizon returns HTTP 500.
Turning on debug mode on /etc/openstack-dashboard/local_settings.py and accessing the dashboard again returns the traceback attached to this report.
Setting USE_I18N to False on local_settings.py fixes this behavior but certainly this is not the better approach to solve it since Horizon cares about internationalization. Perhaps it may be a solution while Horizon does not fix it.
I've successfully reproduced this bug on Newton and master deploying it with kolla.
I've also reported this bug on horizon: https://bugs.launchpad.net/horizon/+bug/1632146
Maybe you can check out this link: https:/
They may be the same thing.