The group header button in the Agenda view is not localized in some Telerik themes.
To work this around, extract the default GroupHeaderTemplateSelector from the Telerik.Windows.Controls.ScheduleView.xaml file. Then replace the Text binding of the second TextBlock in the AgendaViewHorizontalTopDateTemplate. It should be:
<telerik:GroupHeaderTemplateSelector.AgendaViewHorizontalTopDateTemplate>
<DataTemplate>
<ContentPresenter>
<ContentPresenter.Content>
<StackPanel Orientation="Horizontal" VerticalAlignment="Top" Margin="8 0 10 0">
<TextBlock Margin="0 0 7 0" FontSize="40" Text="{Binding Name, StringFormat='{}{0:dd}'}" />
<StackPanel Orientation="Vertical" Margin="0 10 0 0" VerticalAlignment="Top">
<TextBlock Text="{Binding Name, StringFormat='{}{0:dddd}'}" />
<TextBlock Text="{Binding Name, StringFormat='{}{0:MMMM, yyyy}'}" />
</StackPanel>
</StackPanel>
</ContentPresenter.Content>
</ContentPresenter>
</DataTemplate>
</telerik:GroupHeaderTemplateSelector.AgendaViewHorizontalTopDateTemplate>