[Bug]: Search with custom date range shows date picker does not respect user locale (always mm/dd/yyyy)

⚠️ 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 OR Nextcloud Community Forum (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

When using the date pickers in the custom date range in the universal search function (magnifier icon in top right), the date pickers ignores the Language and Region settings, and always shows a date picker with the format mm/dd/yyyy.

This happens on three separate instances. One of them is the one described in this report, the others are Nextcloud 28.0.4 and Nextcloud 29.0.0 (shared hosting).

Steps to reproduce

1. Go to Settings, change Region to Spain or any region where the date is not formatted as mm/dd/yyyy.
2. Click the unified search button (magnifier glass on top bar, right).
3. Click on "Date"
4. Select "Custom date range"
5. See two date fields with mm/dd/yyyy format.

Expected behavior

Date fields should follow the Region setting, in this case, dd/mm/yyyy.

Installation method

Community Manual installation with Archive

Nextcloud Server version

29

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.2

Web server

Nginx

Database engine version

MariaDB

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

Upgraded to a MAJOR version (ex. 22 to 23)

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": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "29.0.0.19",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "default_phone_region": "es",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "twofactor_enforced": "false",
        "twofactor_enforced_groups": [],
        "twofactor_enforced_excluded_groups": [],
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_smtpauthtype": "LOGIN",
        "mail_smtpport": "587",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpauth": 1,
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpsecure": "tls",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "app_install_overwrite": [
            "apporder",
            "music",
            "onlyoffice"
        ],
        "maintenance": false,
        "maintenance_window_start": 1,
        "theme": "",
        "loglevel": 2
    }
}

List of activated Apps

Enabled:
  - activity: 2.21.1
  - bruteforcesettings: 2.9.0
  - calendar: 4.7.4
  - circles: 29.0.0-dev
  - cloud_federation_api: 1.12.0
  - collectives: 2.11.0
  - contactsinteraction: 1.10.0
  - dav: 1.30.1
  - federatedfilesharing: 1.19.0
  - federation: 1.19.0
  - files: 2.1.0
  - files_downloadlimit: 2.0.0
  - files_pdfviewer: 2.10.0
  - files_reminders: 1.2.0
  - files_sharing: 1.21.0
  - files_trashbin: 1.19.0
  - files_versions: 1.22.0
  - logreader: 2.14.0
  - lookup_server_connector: 1.17.0
  - music: 1.11.0
  - notifications: 2.17.0
  - oauth2: 1.17.0
  - onlyoffice: 9.2.0
  - password_policy: 1.19.0
  - photos: 2.5.0
  - previewgenerator: 5.5.0
  - provisioning_api: 1.19.0
  - related_resources: 1.4.0
  - settings: 1.12.0
  - sharebymail: 1.19.0
  - suspicious_login: 7.0.0
  - tables: 0.7.2
  - text: 3.10.0
  - theming: 2.4.0
  - twofactor_backupcodes: 1.18.0
  - twofactor_totp: 11.0.0-dev
  - updatenotification: 1.19.1
  - viewer: 2.3.0
  - workflowengine: 2.11.0
Disabled:
  - admin_audit: 1.19.0
  - comments: 1.19.0 (installed 1.15.0)
  - dashboard: 7.9.0 (installed 7.5.0)
  - encryption: 2.17.0
  - files_external: 1.21.0
  - firstrunwizard: 2.18.0 (installed 2.14.0)
  - impersonate: 1.16.0 (installed 1.16.0)
  - nextcloud_announcements: 1.18.0 (installed 1.14.0)
  - privacy: 1.13.0 (installed 1.9.0)
  - recommendations: 2.1.0 (installed 1.4.0)
  - serverinfo: 1.19.0 (installed 1.15.0)
  - support: 1.12.0 (installed 1.8.0)
  - survey_client: 1.17.0 (installed 1.14.0)
  - systemtags: 1.19.0 (installed 1.15.0)
  - user_ldap: 1.20.0
  - user_status: 1.9.0 (installed 1.5.0)
  - weather_status: 1.9.0 (installed 1.5.0)

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

Can't provide right now, need to clean up sensitive information.

Additional info

No response

Reproduce the issue from the universal search button: choose a non-US Region, open Date, and select Custom date range. Locate the frontend entry point for those two date fields and trace how the displayed format is selected. Done means the fields follow the user's regional format, including the reported Spain case, with coverage for the regression if the area has tests.