It is quite possible to create a customized locale file.
The locale layout is defined in relevant locale file.
The locale file can import other files - so in your case you could create a copy with a slightly different name - then use that - do not modify the system file - it will be overwritten without any warning.
I have no experience with this - read the above linked topic - read the article Locale - ArchWiki
Copy the file to your home
cp /usr/share/i18n/locales/ru_RU ~/my_RU
Then modify - possibly use the en_DK as template or guidance, save the file and copy back
sudo cp ~/my_RU /usr/share/i18n/locales
Make the necessary changes to /etc/locale.gen (run locale-gen) , locale.conf and ~/.config/plasma-localerc accordingly.
There is absolutely no guarantee that you will be able to achieve the desired result - but - boy - you are in for some learning …