I am trying to have everything in English, except day and month, which I want to change to my language. I followed this post, but this command does not work.
defaults write NSGlobalDomain AppleLocale nl_NL
In the comments for the accepted answer in the post to which you linked, WEBjuju states it also requires "fiddling with the date in preferences" to it to truly "take." Have you tried that? Also, another response stated that, as of Sep 2016, Sierra could not do this change that was allowed in El Capitan. Which version do you have?