They explained it in the other thread.
You set “Time” to en_DK.UTF-8.
But what was not clearly communicated in that thread, was that there are 2 different configs (at least on my system).
The settings you change is located in ~/.config/plasma-localerc.
Then there is also the SYSTEM locale, depending on what distro you are on, mine is in /etc/locale.conf.
None of that has anything to do with fonts though.
You should not have to install any fonts if you do above.
Edit
Wait, you want the other way around. Then en_US.UTF-8 is probably what you want (if you want YYYY-MM-DD) or maybe C will work. I’m not sure how KDE will handle C though.