The setting becomes visible when Date is selected as the format type for a field in the General settings. Select which language the date field is written in on the document. (More details.)
This setting is required for dates where the month is written in alphabetic letters on the document instead of a number. For example, January or Jan. instead of 1.
This setting has no effect on how the date field is displayed in Optimize and Verify.
You can also adjust the year on documents for countries where alternate calendars are used.