I require to format my excel sheet using Indian system of formatting number
to use thousand separators which is like ##,##,##,##0.00 and not as
###,###,##0.00.

I have already changed the regional settings in Windows Contol Panel
suitably and checked the suitable option in Excel/Tools/Options to select
thousand separator as per system settings.

Still my number are formatted as ###,###,##0.00

Even custom setting to ##,##,##,##0.00 gets reverted to ###,###,##0.00