Very interesting situation!
You may need to utilize a custom form control to be honest, i am not sure if there is something that can know to switch it back and forth otherwise.
If it's as simple as a textbox with the value in it, you would just overwrite the Value property's Get to do a quick check on the current user's culture (LocalizationContext), and replace commas with periods. You'll have to do some testing to make sure it behaves the way you want to.
If you aren't comfortable making a custom form control, tell us and we can try to help further.
Not sure if there is an In Kentico way of doing this though.