Jira Calender does not support Estinian Language. The customer is getting the below error.

Exception: Uncaught TypeError: Cannot read property 'DEF_DATE_FORMAT' of undefined

The problem here is with the default date picker field in Jira (such as the Start/End Sprint date) that does not support other languages than English.

Checking this directory: $jira_install/atlassian-jira/includes/lib/calendar/lang, there is no language pack for Estonia while some other non-English languages are supported.