Decimal separators on reports are initially controlled by a CSV setting as long as it is not overridden on the specific report definition (RPD).

If this setting is not defined, the default is a period: '.'

To excel - that is, when using 'export to excel' - and not when the file is exported as csv and opened in excel - 

it is formatted as excel format and will follow the 'regional settings' as defined by the user on his/her laptop/pc.