This article describes an issue with registering custom cultures in Episerver DXC.
While it is possible to register a custom culture language locally using the CultureAndRegionInfoBuilder, it doesn't work in Azure WebApp.
The CultureAndRegionInfoBuilder.Register method throws UnauthorizedAccessException.
Since running Episerver DXC as a PaaS WebApp in a multi-tenant environment, this puts certain limitations on what can be changed on the OS level. These limitations do not allow for changes to be made to avoid the UnauthorizedAccessException error at this time. The determination at this time is that this feature is not currently supported.