I wanted the dates on my Kubuntu system to be shown as Dutch dates. Therefore, I needed to add a locale. The place to do it is in /var/lib/locales/supported.d/local. It now contains this: 

en_US.UTF-8 UTF-8
nl_NL.UTF-8 UTF-8

Then run locale-gen to generate the locales.

Set the following environment vars (in profile or something) to let everything behave properly: 

export LANG="nl_NL.UTF-8"
export LC_MESSAGES="en_US.UTF-8"
export LC_COLLATE="C"

I don’t know anymore why I chose this specific configuration. I guess this means that everything, from thousand and decimal separators to dates is Dutch, and messages are English. I don’t know, however, why the collate is C. I vaguely remember something about it otherwise not including some characters in the sorting, causing weird sorting in words with dashes and such.