Please take a look at the configuration file for the Discourse Docker container (probably app.yml) and make sure that DISCOURSE_DEFAULT_LOCALE is commented out. It should look like this:
# DISCOURSE_DEFAULT_LOCALE: en
You won’t be able to change the default_locale within Discourse when that environment variable is set.
@sam I don’t think this is a bug, but it might be a good idea to disable changing the locale the UI (wizard and site settings) if it is shadowed. A hint why it is disabled would be great too. What do you think?