Hi Radoslaw Kubas,
Greetings from Syncfusion.
We don't have direct support for changing the default text of the footer buttons (Cancel, Ok). But you can achieve this requirement by using custom view along with the localization support and created a simple sample for your reference.
Please download the sample from the below link.
Screenshot:
Please let us know if you have to need any other details.
Regards,
Hemalatha M.