When releasing a version from the Project Overview > Releases, you have the option to set the Release Date. When my language is set to Latvian, the date picker for the Release Date is corrupted, as per this screenshot: corrupted_release.png.
As per above, all the values are undefined.
This problem does not happen if you are releasing the version when your language is set to English (US or UK).

As I know in issue JRA-42923 this problem was resolved with other non-English languages, but still in JIRA 6.4.7. this problem appear when JIRA language is set to Latvian.