I've created a new (optional) parameter for the dateselectionwidget velocity macro to specify the date format. I've also created a new VelocityPortletPaneledAction.getDateFormatString() method which dynamically determines the date format based on the user's preferred language/locale.
It looks like the velocity tools which use this date picker are: announcement, assignment, calendar, & content
If I've missed any they will default to the US locale (e.g. no change in behavior). I'll close this issue when the above tools have been updated to use the new parameter and method.
1. Change Language/Locale (Preferences -> Locale) to any non-US locale (e.g. "English (United Kingdom)" ) and verify the date picker modifies the month/date/year order appropriately (e.g. in the UK, the order is day/month/year).