Ninja Forms uses the jQuery UI Datepicker to add datepicker fields to forms. jQuery is bundled with WordPress, so the jQuery UI Datepicker is an easy choice. However, the datepicker defaults still need to be localized. In the short term, I have put together a plugin that helps to localize the datepicker defaults for Ninja Forms. Note: The plugin currently has limited language support based on immediate customer needs, but can be easily updated as those needs change.
Locale and Site Language