I'd assume comma, like en_DK.

I'd like an en_EU definition to be added, except I went to find an official definition[1], and found the EU uses both.

Elsewhere [2] it says the use of a comma is due to "technical limitations".

I use en_GB or en_IE, but

  export TIME_STYLE=long-iso

[1] https://style-guide.europa.eu/en/content/-/isg/topic?identif...

[2] https://commission.europa.eu/system/files/2023-11/styleguide... §6.10.