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.
https://docs.kentico.com/k10/custom-development/developing-form-controls/example-developing-custom-form-controls
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.