Hi Kevin,
I have test the presented scenario and found out that the reason for this issue is that, you could not set the culture property on a Control in asp:Panel initially hiden by Visible="False".
Please try to hide the panels by using CSS class and apply the following:
CSS file:
.hide{
visibility: hidden;
display: none;
}
Give this a try and let us know how it goes.
Greetings,
Maria Ilieva
the Telerik team