User:Omegatron/Date formatting/Language codes

If we were to determine the user's preferred date format based on their browser's preferred language codes, how reliable would this be? What language tags/country codes would map to what?

Date formatting is essentially a feature of language, so this would be an appropriate use of the tags. en-us always formats as mm/dd/yyyy, while en-gb always formats as dd/mm/yyyy, for instance.

(Based on Calendar date and the list here)

January 15, 2001

en (since most visitors are American?)
en-us
en-ca
fr-ca
...

15 January 2001

en-au
en-gb
en-ie
en-za
bg
de
de-ch
el
es
es-mx
fr
fr-ch
he
hi
in
it
nl
nl-be
pl
pt-br
ro
sl
sv
th
...

2001 January 15

2001-01-15

???