The number of languages supported by the standard CJuiDatePicker implementation is limited and the language code to use is different from the application's language code.
This Wiki shows how to improve on that.
When using themes you want to changes some of the translations specifically to the theme while keeping the standard translations for most of the text.
Duplicating the main translation file is a headache for maintainance.
This how to provides a solution.
You would find having such a need in most Yii Projects. Your database needs a specific format to store date fields, while you may want to display it in different formats to the users on forms and other views.