A mockup I’m played around with that tries to support the normal mortal case of just selecting the right locale, but supporting the apparently common situation of
a) wanting just the UI in US English which leads people to effectively set LANG=en_US and then fighting their applications one at a time to get them to not use Letter paper, not use a baffling date format, getting . vs , in numbers, inches vs cm, and so on
b) wanting dates in ISO format
and at the least letting you know up-front the effect your settings will have on applications.
e.g. X wants everything to be standard German
e.g. X wants UI in English because X finds that more comfortable but normally X will be writing in German and using metric, wants the normal German formatting rules to apply for numbers and currency, except for dates because X wants to use the ISO date format to avoid cross-cultural cockups on dates like the 6th of April and the 4th of June.
What maps to what