@TNif the field is a free-form field, the user can type anything they need into that field. However, for Amount fields, the user must enter decimal points. I just tested this with a user that is set up to use Euros. I entered the amount using a comma and the system showed a message that the field requires a numeric value. This tells me the default Amount fields you see on expense reports are hard coded to require amounts be entered with decimals.

Maybe someone else here on the Community has experienced this or has a different answer.