I saw in a lot of AjaxControlToolkit.resources.dll for different languages, even mine (Russian) in my bin folder so I guess that's real to change the language of my Ajax Calendar Extender.

                          <asp:TextBox ID="TextBox4" runat="server" oninit="TextBox4_Init" />
                          <ajaxToolkit:CalendarExtender ID="TextBox4_CalendarExtender" runat="server" 
                              Enabled="True" FirstDayOfWeek="Monday" Format="dd.MM.yyyy" 
                              TargetControlID="TextBox4" />
                          <br />

It's English by default

But how can I change it to my Language ? (or to the current culture language)