Omdat Atrecht niet echt meer word gezegt in het Nederlands dat zou dan old_name:nl=Atrecht moeten zijn

Yes but that would be ideal but the Brussels' dialect doesn't have a ISO code like those Swiss dialects so I thought this would be the best option

I think you right about the duplicate but it would maybe still be helpful to use the names in the dialect because their is a active movement to use the dialect again in daily life and also for historical purposes using the dialect names would be useful