Thanks Rocket and Onemyndseye

I can confirm that both locale and su work in my 5.1.1 system as you predict.

(In 5.1.1) kdesurc is in :-

/etc/skel/.kde/share/config/kdesurc
(Leave unchanged if remastering a live CD to preserve the Knoppix 5.1.1. way)

and, for user(s) in an HD-installed system,

$HOME/.kde/share/config/kdesurc
(Change as per Onemyndseye's posting for each user)

Whew !

PS Just seen your new thread dedicated to locale Onemyndseye : excellent move to break it out with its own title.