/// <summary>
        /// Holds default assembly name
        /// </summary>
        private static string defaultAssembly = ""; 

        /// <summary>
        /// Gets / Sets default assembly
        /// </summary>
        public static string DefaultAssembly 
        { 
            get
            {
                return !String.IsNullOrEmpty(defaultAssembly) ? 
                                                defaultAssembly : 
                                                Instance.GetAssemblyName(System.Reflection.Assembly.GetExecutingAssembly());
            } 
            set
            {
                defaultAssembly = value;
            }
        }

        /// <summary>
        /// Holds default dictionary name
        /// </summary>
        private static string defaultDict = "";

        /// <summary>
        /// Gets / Sets default dictionary
        /// </summary>
        public static string DefaultDict 
        {
            get
            {
                return !String.IsNullOrEmpty(defaultDict) ? defaultDict : ResourcesName;
            }
            set
            {
                defaultDict = value;
            }
        }

        /// <summary>
        /// Parses a key ([[Assembly:]Dict:]Key and return the parts of it.
        /// </summary>
        /// <param name="inKey">The key to parse.</param>
        /// <param name="outAssembly">The found or default assembly.</param>
        /// <param name="outDict">The found or default dictionary.</param>
        /// <param name="outKey">The found or default key.</param>
        public static void ParseKey(string inKey, out string outAssembly, out string outDict, out string outKey)
        {
            // reset the vars to null
            outAssembly = null;
            outDict = null;
            outKey = null;

            // its a assembly/dict/key pair
            if (!string.IsNullOrEmpty(inKey))
            {
                string[] split = inKey.Trim().Split(":".ToCharArray(), 3);

                // assembly:dict:key
                if (split.Length == 3)
                {
                    outAssembly = !string.IsNullOrEmpty(split[0]) ? split[0] : null;
                    outDict = !string.IsNullOrEmpty(split[1]) ? split[1] : null;
                    outKey = split[2];
                }
                
                // dict:key
                // assembly = ExecutingAssembly
                if (split.Length == 2)
                {
                    outAssembly = !String.IsNullOrEmpty(DefaultAssembly) ? DefaultAssembly : null;
                    outDict = !string.IsNullOrEmpty(split[0]) ? split[0] : null;
                    outKey = split[1];
                }
                
                // key
                // assembly = ExecutingAssembly
                // dict = standard resourcedictionary
                if (split.Length == 1)
                {
                    outAssembly = !String.IsNullOrEmpty(DefaultAssembly) ? DefaultAssembly : null;
                    outDict = !String.IsNullOrEmpty(DefaultDict) ? DefaultDict : null;
                    outKey = split[0];
                }
            }
            else
            {
                // if the passed value is null pr empty, throw an exception if in runtime
                if (!Instance.GetIsInDesignMode())
                {
                    throw new ArgumentNullException("inKey");
                }
            }
        }

        /// <summary>
        /// Gets or sets the name of the Assembly where the .resx is located.
        /// If it's null, the default assembly (where this LocalizeEngine is located at) will get returned
        /// </summary>
        public string Assembly
        {
            get
            {
                return this.assembly ??
                       LocalizeDictionary.DefaultAssembly;
            }

            set { this.assembly = !string.IsNullOrEmpty(value) ? value : null; }
        }

        /// <summary>
        /// Gets or sets the Name of the .resx dictionary.
        /// If it's null, default will get returned
        /// </summary>
        public string Dict
        {
            get { return this.dict ?? LocalizeDictionary.DefaultDict; }

            set { this.dict = !string.IsNullOrEmpty(value) ? value : null; }
        }

WPFLocalizeExtension.Engine.LocalizeDictionary.DefaultAssembly = "DefaultAsembly";
WPFLocalizeExtension.Engine.LocalizeDictionary.DefaultDict = "DefaultDictionary";

<TextBlock Name="TextName" Text="{loc:LocText myTextKey}"/>

<Window x:Class="AssemblyTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Engine="clr-namespace:WPFLocalizeExtension.Engine;assembly=WPFLocalizeExtension"
        Engine:LocalizeDictionary.DesignCulture="de"
        Engine:LocalizeDictionary.DefaultAssembly="AssemblyTest"
        Engine:LocalizeDictionary.DefaultDictionary="Strings"
        xmlns:lex="clr-namespace:WPFLocalizeExtension.Extensions;assembly=WPFLocalizeExtension"
        Title="MainWindow" mc:Ignorable="d" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" d:DesignHeight="114" d:DesignWidth="248" SizeToContent="WidthAndHeight">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />               
        </Grid.RowDefinitions>
        <Label Grid.Row="0" Content="{lex:LocText TestText}" Height="28" HorizontalAlignment="Center"/>
        <StackPanel Name="TestPanel" Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center" Engine:LocalizeDictionary.DefaultDictionary="Strings2">
            <Button Content="{lex:LocText ButtonDE}" Height="23" Width="75" Click="ButtonDE_Click" Engine:LocalizeDictionary.DefaultAssembly="AssemblyTestResourceLib" />
            <Button Content="{lex:LocText ButtonEN}" Height="23" Width="75" Click="ButtonEN_Click" />
        </StackPanel>
        <Button HorizontalAlignment="Center" Name="ButtonAssembly" Content="Assembly" Grid.Row="2" Height="23" Width="75" Click="ButtonAssembly_Click" />
    </Grid>
</Window>

Here's the file. I'm just doing a proof-of-concept at this point, to see how easy it is to swap languages on the fly. 
I really like the ability to set the default assembly and dictionary, so the "binding" isn't as cluttered.

<Window x:Class="LanguageTester.MainWindow"
		xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
		xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
		xmlns:localizeEngine="clr-namespace:WPFLocalizeExtension.Engine;assembly=WPFLocalizeExtension"
		xmlns:localExt="clr-namespace:WPFLocalizeExtension.Extensions;assembly=WPFLocalizeExtension"
		localizeEngine:LocalizeDictionary.DesignCulture="en"
		localizeEngine:LocalizeDictionary.DefaultAssembly="LanguageTester"
		localizeEngine:LocalizeDictionary.DefaultDictionary="Strings"
		Title="MainWindow"
		Height="350"
		Width="525">
	<Grid>
		<Grid.RowDefinitions>
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
			<RowDefinition />
		</Grid.RowDefinitions>
		<Button Grid.Row="0"
				Content="{localExt:LocText Button}" />
		<TextBlock Grid.Row="1"
				   HorizontalAlignment="Center"
				   VerticalAlignment="Center"
				   Text="{localExt:LocText TextBlock}" />
		<Label Grid.Row="2"
			   HorizontalAlignment="Center"
			   VerticalAlignment="Center"
			   Content="{localExt:LocText Label}" />
		<GroupBox Grid.Row="3"
				  HorizontalAlignment="Stretch"
				  VerticalAlignment="Stretch"
				  Header="{localExt:LocText GroupBox}" />
		<CheckBox Grid.Row="4"
				  HorizontalAlignment="Center"
				  VerticalAlignment="Center"
				  Content="{localExt:LocText CheckBox}" />
		<RadioButton Grid.Row="5"
					 HorizontalAlignment="Center"
					 VerticalAlignment="Center"
					 Content="{localExt:LocText RadioButton}" />
		<ComboBox x:Name="myComboBox"
				  Grid.Row="6"
				  HorizontalAlignment="Center"
				  VerticalAlignment="Center"
				  MinWidth="150" />
		<Button Grid.Row="7"
				Content="Change Language"
				Click="Button_Click" />
	</Grid>
</Window>

-Dan