Hi!
Out of the box it seems the plone datepicker (pat-pickadate) uses am/pm timeformat, as well as a dateformat like "marts 17, 2018".
(The month has been translated correclty, but the format is wrong - correct would be "17. marts 2018")
The date and time formats for the datepicker pattern are set i translation files for the plone domain:
pickadate_date_format and pickadate_time_format
It is a little disorienting that
a) The usual date_format_long and date_format_short (used in listings etc.), and all the strings for months and days, are defined for the plonelocales domain - the datepicker translations has to be in the plone domain
b) The format strings are different - date_format_long and date_format_short uses strftime formating, while the datepicker formats uses some javascript based format (which one? does not seem to be similar to jquery ui datepickerhttp://api.jqueryui.com/datepicker/#utility-formatDate )
by hacking around this format string solved the issue for me: