I recommend don't change those settings in Web.config, see the article Forcing a Specific Language for the correct way to specify the language settings that the thread will execute as.
Setting it like you did in Web.config can cause null exceptions because the .cz resources are incomplete, and if it tries to use a missing key it can cause a null reference exception. Doing it the way I recommend it will always fall back to English for any missing keys and the language you set it to.
You'll also need to add these properties on <asp:ScriptManager in your layout.master file
EnableScriptGlobalization="true" EnableScriptLocalization="true" ie like this:
<asp:ScriptManager ID="ScriptManager1" EnablePageMethods="true" runat="server" EnableScriptGlobalization="true" EnableScriptLocalization="true" />
Hope that helps,