We recognized that in out Jira 5.2.6 the calendar weeks in the standard calendar do not count up correctly when using the german language. It's not even conform to ISO 8061, which is set as standard. The same bug seems to appear in spanish and french. So maybe all languages that have monday as first day of the week.

Customer support opened a bug for us at: https://jira.atlassian.com/browse/JRA-31934.
But my question now is:
Has anyone successfully worked around that error? I'm pretty stuck at the moment. And changing our first day of the week is not a viable option atm.