In my opinion a new dateFormat between short and medium is needed for the datepicker to work correctly.
Now there is
'full' => 'EEEE d MMMM yyyy', 'long' => 'dd MMMM yyyy', 'medium' => 'dd/MMM/yyyy', 'short' => 'dd/MM/yy',
and a ‘dd/MM/yyyy’ is needed and that’s way:
if you set the property "language" of the datepicker it sets (in italian language for example) a date of 4-digit year and 2-digit (numerical) month:
eg: 01/01/2009 and yii doesn’t have this format!
you can call it "small" for example.
otherwise the language property is useless in this scenario.