Why does my MacBook not display the date and time in Welsh? In "Languages and Region" I have set my only language to be Cymraeg (Welsh).  It is the only one in the list of preferred languages. However, the date in the top bar is in english. 

If I change the language to French it displays the date and time in French. My iPhone can display it in Welsh... 

Any ideas?
Try using this command in Terminal:

defaults write NSGlobalDomain AppleLocale -string cy_GB

It worked for me in 10.13, but have not tested in later MacOS versions.

This question indicates it may no longer work.

Mojave and its non-respect of the AppleLocale preference