incomplete but functional solution:
fist check the output of $ locale
then read the manuals of these packages and install if suited:
locale, localedef, localehelper, locales-all, and localepurge
I only installed locales-all and set this in my .bashrc
export LANG="en_GB"
export LANGUAGE="en_GB:en"
export LC_CTYPE="en_GB"
export LC_NUMERIC="de_DE"
export LC_TIME="de_DE"
export LC_COLLATE="de_DE"
export LC_MONETARY="es_MX"
export LC_MESSAGES="en_GB"
export LC_PAPER="de_DE"
export LC_NAME="de_DE"
export LC_ADDRESS="de_DE"
export LC_TELEPHONE="de_DE"
export LC_MEASUREMENT="es_MX"
export LC_IDENTIFICATION="de_DE"
export LC_ALL=
upon next log-in, my sytem was as desired.
with this I have an overhead: unused locales stored in arround 250MiB (as far as I could extract from ncdu). Since Storage is cheap nowadays, I will keep these for now.