During test deployments of my Magento 2 setup I continually ran into problems if I used a language other than en_US.
After running the setup and a 'bin/magento setup:static-content:deploy en_GB' the front end files would display just fine but the admin interface was missing all the CSS etc.
I eventually traced the problem back to how the initial admin user is created. Regardless of what language is chosen during setup the admin user is always setup with the interface locale as en_US. To me this seems counter intuitive, the initial admin user should be configured with the same language as the store configuration.
Changing the admin user interface locale to en_GB in the database (not recommended) solves the problem instantly and the admin interface displays as expected.
If the initial admin user was correctly configured this would have saved me time and I have no doubt other people are trudging around in circles try to resolve what looks like a complex issue but is in fact a simple configuration problem.