Indeed, setting "em" with decimal values will not work for cultures where the decimal separator is ",". I am logging this as a bug, so that the scenario could be further investigated by our developers team.
Currently, the only thing that I could suggest is to either use integer values for the width or replace the command with dot on client-side.
Please excuse us for any inconvenience caused by this.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.