[Bug]: locale not displayed correctly

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Issue relates to the UI preview (https:///settings/user), not sure about the rest of the system.

The Nextcloud documentation refers to Momentjs locales.
When I check the e.g. 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:

1. the format shown for a country seems to change / is not updated in the UI
   when I tried just now Canada showed as a preview “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.

2. 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 German momentjs definition all the settings except the language related ones and from the English momentjs definition all the language related ones (names of days/months/relative time).

Steps to reproduce

1. go to the user settings (https:///settings/user)
2. select / change the locale

Expected behavior

see bug description

Installation method

Official Docker image

Operating system

No response

PHP engine version

No response

Web server

No response

Database engine version

No response

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "htaccess.RewriteBase": "\/",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "apps_paths": [
            {
                "path": "\/var\/www\/html\/apps",
                "url": "\/apps",
                "writable": false
            },
            {
                "path": "\/var\/www\/html\/custom_apps",
                "url": "\/custom_apps",
                "writable": true
            }
        ],
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "overwritehost": "...",
        "overwriteprotocol": "https",
        "trusted_domains": [
            "...",
            "..."
        ],
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "23.0.3.2",
        "overwrite.cli.url": "...",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpauth": 1,
        "mail_smtpauthtype": "LOGIN",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "465",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpsecure": "ssl",
        "default_phone_region": "...",
        "maintenance": false,
        "enable_previews": true,
        "simpleSignUpLink.shown": false
    }
}

List of activated Apps

Enabled:
  - accessibility: 1.9.0
  - activity: 2.15.0
  - bruteforcesettings: 2.3.0
  - calendar: 3.2.2
  - circles: 23.1.0
  - cloud_federation_api: 1.6.0
  - comments: 1.13.0
  - contactsinteraction: 1.4.0
  - dashboard: 7.3.0
  - dav: 1.21.0
  - federatedfilesharing: 1.13.0
  - federation: 1.13.0
  - files: 1.18.0
  - files_external: 1.15.0
  - files_pdfviewer: 2.4.0
  - files_rightclick: 1.2.0
  - files_sharing: 1.15.0
  - files_trashbin: 1.13.0
  - files_versions: 1.16.0
  - files_videoplayer: 1.12.0
  - firstrunwizard: 2.12.0
  - logreader: 2.8.0
  - lookup_server_connector: 1.11.0
  - maps: 0.1.10
  - nextcloud_announcements: 1.12.0
  - notifications: 2.11.1
  - oauth2: 1.11.0
  - password_policy: 1.13.0
  - photos: 1.5.0
  - privacy: 1.7.0
  - provisioning_api: 1.13.0
  - recommendations: 1.2.0
  - richdocuments: 5.0.3
  - richdocumentscode: 21.11.204
  - serverinfo: 1.13.0
  - settings: 1.5.0
  - sharebymail: 1.13.0
  - support: 1.6.0
  - systemtags: 1.13.0
  - text: 3.4.1
  - theming: 1.14.0
  - twofactor_backupcodes: 1.12.0
  - twofactor_totp: 6.2.0
  - twofactor_u2f: 6.3.0
  - updatenotification: 1.13.0
  - user_status: 1.3.1
  - viewer: 1.7.0
  - workflowengine: 2.5.0
Disabled:
  - admin_audit
  - contacts: 4.1.0
  - encryption
  - mail: 1.11.7
  - spreed: 13.0.4
  - survey_client: 1.11.0
  - user_ldap
  - weather_status: 1.3.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

[08/Apr/2022:11:21:22 +0000] "PUT /ocs/v2.php/cloud/users/userxyz HTTP/1.1" 200 922
[08/Apr/2022:11:21:22 +0000] "GET /ocs/v1.php/cloud/user?format=json HTTP/1.1" 200 1307

Additional info

No response

Start by reproducing the locale preview issue at /settings/user, then compare the Nextcloud language-configuration documentation with the referenced Moment.js locale definitions. Trace the locale handling from the user-settings entry point; done means country-specific date, time, and week settings display correctly while language-specific names remain appropriate.