Using Nextcloud v23.
What locale to choose to get the following?
(https://mynextcloud/settings/user)
E.g. English +
Somehow you have already answered your question yourself 
Or maybe I don’t quite understand what your question is…
I’m not aware of any way to set the format of the date independently of the locale setting, someone please correct me if there is one. So afaik you have no choice but to try and find a locale, which just displays the format of the date the way you want and if possible everything else like currency (is this even used somewhere inside the Nextcloud UI?) etc… too. Or in other words, the guys and gals that are maybe from the US and now are working in Europe, eventually have to get used to the fact that the date is displayed as it is in Europe.
Except for the error you found with the “Europe” setting, the general idea of these settings is, that you can set the language for the interface and then select the country where you are in or where you are working. For example if an American is working in Germany, he or she can have the interface in English, but still use the correct formats for Germany, by selecting German (Germany) in the locale setings.
Don’t think I’m answering my own question.
The Nextcloud documentation refers to Momentjs locales.
When I check the UK momentjs locale, it seems to be close to the one I am looking for: dd-mm-yyyy, 24h, Monday. But, it is now shown/used as expected.
There are multiple issues:
the format shown for a country seems to change / is not updated in the UI
when I tried just now Canada showed “yyyy-mm-dd 12h Sunday” (before: Monday),
when I select Netherlands it shows 12h even though the momentjs locale is 24h,
when I select UK it shows up as dd/mm/yyyy 12h in the Nextcloud UI, instead of the momentjs dd-mm-yyyy 24h format.
the Nextcloud locale combines language and format, but the momentjs locale is for one specific language
E.g. momentjs Germany shows the days/months/relative time in German. So when I select in Nextcloud “English (Germany)” I expect Nextcloud picks from the momentjs definition all the settings except the language related ones.
Maybe this is beacause both dd/mm/yyyy and dd-mm-yyyy 12h/24h are perfectley valid notations in UK, at least according to wikipedia, asuming we are talking about the the “English (United Kingdom)” setting in Nextcloud which will give you 24h notation for the time but still slashes instead of dashes for the date. I couldn’t find an English setting with dashes, which is unfortunate but at least not wrong.
But I agree that many of the settings like e.g "English (Germany) or (Europe) do not seem to pick up the correct notations. For example “English (Germany)” should give you 07.04.2002 13:40, at least according to my understanding of how it is supposed to work, which may be wrong…?
Maybe you could file an issue on Github, if you are convinced that there is something wrong or if you have suggestions for a better UI. The current UI is at least confusing, I won’t aruge with that
The Nextcloud documentation states it uses the momentjs locales,
and the Nextcloud UI is not showing it per the momentjs locale definition.
That’s why I wrote what the two issues are.
Yes, the Germany momentjs locale states DD.MM.YYYY.
Nextcloud needs to have a setting option for ISO 8601 date/time standard. I can’t believe this wasn’t a consideration when making a calendar app.
Absolutely, the idea of using your location to set your date format is just strange.
Do I switch date formats whenever I visit another country? No. When I’m scheduling a conference call with people in multiple countries to I write out the dates/times in EVERY location’s formats? No, I just always use ISO 8601. Being FORCED to use any other format by software is frustrating, and disappointing.