As per 

https://community.atlassian.com/t5/Jira-questions/System-language-and-date-format-makes-release-management-a/qaq-p/685428

https://community.atlassian.com/t5/Jira-questions/Finnish-language-pack-date-input-bug/qaq-p/715331

There is an issue with System language and date format 

The system date format is d/MMM/yy, which is completely fine in english pack as the date is in that format from the calendar app. The calendar outputs it as "30/JAN/18" and is recognized by the system.

The Finnish language pack calendar outputs it as "30/Tammi/18" and the issue creator gives an error about this. The only way is to get it to work is to manually enter the date in the format required.