Datetime format is not correct when using Java 8

Steps to reproduce the problem:

1. Use Java 8.
2. Install JIRA-6.3.15-language-pack-ru_RU.jar to JIRA via https://translations.atlassian.com/dashboard/download?lang=ru_RU#/JIRA/6.3.15
3. Change user profile to a Russian language.
4. Edit date time picker value to 7/май/15 10:17 PM and get this error message:
   
5. Strangely, when changing date time picker value to 7/мая/15 10:17 PM, and it's working.

The problem is not reproducible when JIRA using Java 7.

Questions:

1. Why the JIRA set the 'd/май/yy' format if it's wrong ?
2. Why and when setting 'd/мая/yy' is working ?