In some Java locales, the date picker sets the date to a format which JIRA cannot parse, returning the error:

You did not enter a valid date. Please enter the date in the format "d-MMM-yy".

This happens for the following languages:

ko (adds date in '23-8-2005' format)
big5 (enters in format 17-八月-2005)

the following languages work:

jp (uses English date format)
zh
tr
ru
sv
(etc - haven't tested them all)