Disclaimer: I'm not certain whether the problem lies in JIRA or actually in the german localization bundle.

The Bug:
Since the update from JIRA 6.0.3 to JIRA 6.3 one of our clients is encountering a problem with the date picker on all occurrences inserting the wrong abbreviation for the month March leading to the error message:
"You did not enter a valid date. Please enter the date in the format "d/MMM/yy", eg. "8/Jan/15""

Using the german loca for JIRA, the correctly formatted date for i.e. 1st of March (German: "März") would be: "1/Mär/15"
Entering this manually is accepted by JIRA.
However, using the JavaScript Date-Picker, it enters "1/Mrz/15" in the respective field, which is denied by JIRA with the error message mentioned above.

Solution:
The JavaScript of the Date-Picker should be corrected in order to use the appreviation "Mär" for March.
Alternatively it could also make sense to change the abbreviation to "Mrz" in JIRA - the date-picker would then already use the correct value.

If you require more info, feel free to let me know.