In JIRA, if the locale in Admin -> General Configuration is set to None, JIRA uses the system locale. When JIRA does not have a translation for the system locale (af, al, ca, du, fi, hr, hu, ko, lt, lv, nl, ro, si, sk, sl, sp, sr, sv, tr), JIRA defaults to English. This is not very useful, because English dates will not validate in the default date format.