Hello Christian,

Currently messages option dedicated to the ActionSheet texts displayed in the adaptive DatePicker are not exposed. Thus, the issue could not be considered as a bug, but a Feature Request. I am changing the type of the issue accordingly. I am also changing the status of the item to 'Unplanned'. We will keep monitoring the popularity of the item, and based on it, the issue could be considered for implementation.

In the meantime, in order to change the needed text, you can handle the activate event of the ActionSheet displayed for the DatePicker. In the event handler you can find the needed element and change its text:

 $('.k-actionsheet-container .k-popup').data('kendoActionSheet').bind('activate', function(w){        
            $('.k-actionsheet-container .k-actionsheet-titlebar .k-text-center:eq(0)').text('Changed title')
        }) 

Here is a Dojo example where this is demonstrated:  - https://dojo.telerik.com/@NeliKondova/AyaJUZub

I hope this helps. 

Regards,
Neli
Progress Telerik