Hi!
Right now there is no particularly straightforward means of changing this, though we’re working on making datepicker formats more customizable and that ought to be ready in an upcoming release, so hang tight!
To clarify though, we’re not particularly preferring the US format over any other (in fact, yyyy-mm-dd is more of a “computer format” than a local one – in the US mm/dd/yyyy is typically preferred) and the fundamental challenge is that it is a quite complex thing to reimplement the complete range of PHP date and time formats (which are used by WordPress) in a Javascript component such as a datepicker.
Bear with us though and hopefully some changes we’re working on will cascade down to Community Events and improve the situation some 🙂