It appears that the language translation for the months of the calendar are not changing according to the user's language settings in their Profile.

To recreate the issue:

1. Set the OS environment LANG to something like German: "export LANG=de_DE" (Or you can set JIRA's Default Language to something like Russian.)
2. Start JIRA
3. Verify Calendar months appear in German when using the Date Picker.
4. Login with another user and specify in their Profile the language to English (US). The site should show English when the user logs in. However, when creating an issue and using the date picker, the months shown are still in German.