D8 ships with localized versions of jQuery UI thanks to the issue in https://www.drupal.org/project/drupal/issues/507502 → .
It would be nice if the datepicker would use the localization based on Drupal's settings (similarly as it currently uses the first day of the week setting from Drupal's regional conf, which btw is currently impossible to be overridden by the webform's CSS/JS setting as the Drupal's setting is always set last, but that could be a different issue (if at all)).
It seemed to me that most of the datepicker's localization files used the same convention as Drupal does, so for example we could include datepicker-et.js for Estonian translation as Drupal uses the et code for Estonian language.
Closed: works as designed
5.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Anyone hitting this:
For a localized date picker , due to a 30+ month regression , you will want jq_ui, a drop in replacement for jquery_ui https://drupal.org/project/jq_ui
Easy replacement no patch required see instructions in the release notes. jq_ui includes a fixed copy of the jquery_ui module , I did a writeup on composer replace and fork https://a1.11pro.ca/en/public-documentation/drupalorg/dx-composer-replac...
Where there's a will, there's a way!