@vayu– Piklist uses the jQuery UI datepicker which is built into WordPress.

You need to pass your translated variables through the options parameter. Here’s an example in French:

  piklist('field', array(
    'type' => 'datepicker'
    ,'field' => 'my-date-field'
    ,'label' => 'Date'
    ,'options' => array(
      'closeText'=>'Fermer',
      'prevText'=>'Précédent',
      'nextText'=>'Suivant',
      'currentText'=>'Aujourd\'hui',
      'monthNames'=>['Janvier','Février','Mars','Avril','Mai','Juin','Juillet','Août','Septembre','Octobre','Novembre','Décembre'],
      'monthNamesShort'=>['Janv.','Févr.','Mars','Avril','Mai','Juin','Juil.','Août','Sept.','Oct.','Nov.','Déc.'],
      'dayNames'=>['Dimanche','Lundi','Mardi','Mercredi','Jeudi','Vendredi','Samedi'],
      'dayNamesShort'=>['Dim.','Lun.','Mar.','Mer.','Jeu.','Ven.','Sam.'],
      'dayNamesMin'=>['D','L','M','M','J','V','S'],
      'weekHeader'=>'Sem.',
      'dateFormat'=>'dd/mm/yy',
      'firstDay'=>1,
      'isRTL'=>false,
      'showMonthAfterYear'=>false,
      'yearSuffix'=>''
    )
  ));

You may also be able to load one of these translation files, though I haven’t tried it.

@vayu– The jQuery docs seem to be lacking. Honestly, I Googled it and found the answer. 😉

@vayu– The Piklist options for the datepicker are the same for every Piklist field. We try not to duplicate information from other codexes, and parameters that go in the options array are really being created by JQuery UI, not Piklist.

Does that make sense?

In most cases, with the exception of validation and sanitization, the options array is not determined by Piklist, but by the underlying library. jQuery UI fields… the WordPress media uploader… etc.

But we do this with our documentation everywhere (and probably should explain it better). For instance, when registering a Post Type using Piklist, we only document the unique Piklist parameters, but you can also use every WordPress parameter in register_post_type as well.

Thank you for letting us know we need to explain it better 😉