Datepicker month parsing is broken

The problem is with the following parsing method in calendar.jsp:

Date.parseDate = function(str, fmt) {
...
	var a = str.split(/\W+/);
...

Problem is that \W does not recognize characters such as á, ä, č, ď, é, í, ĺ, ľ, ń, ó, ô, ŕ, ř, š, ť, ú, ý, ž, etc. as valid characters for words and treats them as characters that separate words, hence the date string such as "19/máj/05" (19/May/2005) is split into ["19", "m", "j", "05"].

Under normal circumstances, parsing should result in ["19", "máj", "05"] array, which can be mapped as

However, incorrect parsing continues and causes incorrect mapping:

As an end result 19/May/2005 pops up in the calendar as 19/Mar/2007.

This was replicated when looking at JRA-8703. Run JIRA with Slovak (Slovakia) as default language (Admin -> General Configuration) or -Duser.language=SK from command line.

I am not sure what is the best way of fixing this.