This is not something I know a huge amount about, but in lieu of any other replies so far…
I think adding the default locale to your app.yml overrides whatever you set for default locale in your site settings (and also removes it from the setting page). But that would only be the default, and you can let people choose (or let their browsers choose) a more suitable alternative for them.
Hopefully that’s relevant, if not this can just be a helpful bump to see if we can rustle up a more knowledgeable response. 
