Moved from https://jira.atlassian.com/browse/CONF-23136 which was filed by Leila Tite:
The Team Calendars plugin does not recognize the global language setting of US English. The suggestion from support was to have each of the 500 Confluence users individually set their language preference to US English. This is not practical and seems unnecessary, since the global language setting has already been changed to US English.
I'm not sure if this only affects the Team Calendar plugin/macro, but that option wasn't available to select as a component.