Setting dateFormat for datepicker should not be translated

File /core/modules/locale/locale.datepicker.js, line 78:
dateFormat: Drupal.t('mm/dd/yy'),

This line sets a localized value for the setting dateFormat for jquery UI datepickers. However this setting should not be trnslated at all, as it should only use a give set of format characters (like the php date() function). See the jquery ui documentation: http://api.jqueryui.com/datepicker/#option-dateFormat

To solve just remove this line from this file in the locale module. If it should get a setting from Drupal at all, it should not be done in the locale module.

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These settings look erroneous to me as well:
- firstDay (line 79): is not a locale specific setting but a system wide setting, and thus should be set elsewhere (if it should be set at all).
- isRTL (line 80): should get a correct setting based on the current language.

Possibly related issues:
- #1761826: Add Date Format data to Drupal.settings.
- #507502: (needs documentation) Provide Locale support for jQuery UI (the issue that introduced or did not solve this and still needs documentation)