How can I change locale settings (datetime, currency,...)

[Jef Driesen]

How can I change the locale of my entire system and for one user only? I 
already know how to use the LC_* environment variables for applications 
launched from a shell (e.g export LC_TIME="nl_BE.UTF-8"). But how can I 
make this change for the entire desktop?

I want to have numbers, date/time and currency in Dutch (Belgium) 
format, but the language should remain English.

And what if I want to customize my system even more. For instance the 
date format with LC_TIME="nl_BE.UTF-8" is DD-MM-YY, but I would like to 
change that to DD-MM-YYYY.