A couple of things..
I've looked at your video but was a little confused. You set the CurrentUICulture although given you don't have the french translation for the scheduler in your project the strings aren't localised. However, as you point out in the video, the dates etc ARE localised. This suggests that you are setting the CurrentCulture somewhere else (and setting it to a french specific culture such as fr-FR) OR maybe you've got your computer's locale set to fr-FR or similar.
Also, you appear to be changing the CurrentUICulture in the Page_Load which I believe is incorrect. Should it not be done in the InitializeCulture?
In any case, I relooked at this on my end by modifying one of the example scheduler projects but was not able to reproduce the problem. What confuses me is that if I were to try set the CurrentCulture to a non-specific culture it would throw an exception there and then, not wait until it was in the bowels of the Scheduler create code (as per the stack trace from the exception I am getting):
[NotSupportedException: Culture 'fr' is a neutral culture. It cannot be used in formatting and parsing and therefore cannot be set as the thread's current culture.]
System.Globalization.CultureInfo.CheckNeutral(CultureInfo culture) +7484386
System.Globalization.DateTimeFormatInfo.GetInstance(IFormatProvider provider) +87
System.DateTime.ToString(String format, IFormatProvider provider) +23
Telerik.Web.UI.Scheduler.Views.Month.RendererBase.CreateNavigationPane(Control container) +158
Telerik.Web.UI.RadScheduler.CreateChildControls(Boolean bindFromDataSource) +195
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +842
From the call stack, it suggests that there is a call to DateTime.ToString(string, IFormatProvider) in Telerik.Web.UI.Scheduler.Views.Month.RendererBase.CreateNavigationPane that is using the Thread.CurrentThread.CurrentUICulture instead of the CurrentCulture?
I will continue to try and distinguish the difference between my project and the modified sample and let you know if I find anything.