Set different display and format language on macOS Sierra

It appears this can be done from the command line. For example Dutch format language (the nl part) with Netherlands (the NL part) set as the Region, even with English as the UI language, should result when you execute this in Terminal:

defaults write NSGlobalDomain AppleLocale -string nl_NL

This was found at tidbits.com.

I used to have different time and fate format language (Spanish) than the main or OS Language (English) in El Capitan just like you, and I spoke with Apple Support regarding this issue and they told me that at the moment those changes are the current updated settings. There is a possibility that third party software can do those changes as El Capitan was able to do but unfortunately Sierra at the moment cannot do that change.

They told me as well that usually with things like this, with so many people depending on it, most of the time it will come back with updates but just can't guarantee when.

So, right now the option is downgrade to El Capitan if you relay on this configuration on a daily basis, or change the OS to the language that you need your time and configuration settings.