Locale en_NL
I wanted a custom locale for my desktop. Most stuff is straight from en_US, but I've made some modifications.
Most relevantly, the decimal separator is a period and thosuand separator a comma.
Installing
On Fedora 39, copy the file en_NL to /usr/share/i18n/locales
Then, compile and select it:
sudo localedef --no-archive -i en_NL -f UTF-8 en_NL.UTF-8
sudo localectl set-locale en_NL.UTF-8
gsettings set org.gnome.system.locale region "en_NL.utf8"
Then logout and log back in and everything should be changed.
Thanks
Many thanks to this answer on Unix Stack Exchange