Are there any methods to customize datepicker in Sulu via XML page config (e.g. I need to change date format)?
Sounds trivial, but http://docs.sulu.io/en/latest/reference/content-types/date.html says "No parameters available."
I am afraid to tell you that this is currently not possible... It would be nice if you create an issue on github also explaining the parameters you'd like to have.
And apart from that, the date format is based on the user language, so if the user has english as UI language he also gets the english formatted date. On the website you can use e.g. the date twig filter to format the date however you like.