problem with resource dictionary

if I write like this in App.xaml
 <Application.Resources>
        <Style x:Key="CustomWindow" TargetType="{x:Type Window}">
        </Style>

        <ResourceDictionary x:Key="lng" Source="lang/eng_lang.xaml">
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="lang/eng_lang.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>

    </Application.Resources>
   then when I use  <Label Content="{DynamicResource SelUn}" the SelUn cannot be resolved.

If I remove
    <Style x:Key="CustomWindow" TargetType="{x:Type Window}">
        </Style>
and it will be only :
 <Application.Resources>
        <ResourceDictionary Source="lang/eng_lang.xaml">
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="lang/eng_lang.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

<Label Content="{DynamicResource SelUn}"  shows the correct string but I need use others styles in app.xml
How can I point into my code for <Label Content= the x:Key for the resource dictionary? I think this is the problem.