Does Anyone know where is the default FORMAT COME FROM ?

A customer would like to use the default date format in NAV (18/04/2016) based on The Windows Regional format setting.
The Windows Regional setting for English(Australia) is d/MM/yyyy (18/04/2016). So I changed the regional setting to 'dd-MMM-yyyy' (18-Apr-2016) and expected NAV to pick it up but it does not.

Current Regional Setting Windows Server 2012 R2:
English(Australia)
Short Date "dd-MMM-yyyy" (Windows displays it correct as 18-Apr-2016)

NAV 2013
On Page display: 18-04-2016
On Report Display: 18/04/2016 (no format function used)


But when I changed to English(United States), Report Date Format changed.
Then I go to change Format (Short Date "dd-MMM-yyyy"), it doesn't work .. it only follow previous FORMAT

Does Anyone know where is the FORMAT COME FROM ?