Eu tenho um UserControl que é usado em um controle pai dessa maneira:

<Views:TranslationTextInput  Translation="{Binding SelectedEntity.Name}"/>

O controle pai DataContext é um ViewModel contendo a propriedade SelectedEntity.

No meu filho UserControl eu defino um novo ViewModel como o DataContext:

    <UserControl.DataContext>
      <vm:TranslationTextInputViewModel x:Name="vm"></vm:TranslationTextInputViewModel>
    </UserControl.DataContext>

No código por trás eu tenho:

    public static readonly  DependencyProperty TranslationProperty = DependencyProperty.Register("Translation", typeof(Translation),typeof(UserControl));

    // .NET Property wrapper
    public Translation Translation
    {
        get { return (Translation)GetValue(TranslationProperty); }
        set { SetValue(TranslationProperty, value); }
    }


 public TranslationTextInput(){
     InitializeComponent();
     DataContext = new TranslationTextInputViewModel();
     SetBinding(TranslationProperty,   new Binding { Path = new PropertyPath     ("Translation"), Mode = BindingMode.OneWayToSource });

Quando executado recebo um erro de ligação:

System.Windows.Data Error: 40 : BindingExpression path error: 'SelectedEntity' property not found on 'object' ''TranslationTextInputViewModel' (HashCode=49954236)'. BindingExpression:Path=SelectedEntity.Name; DataItem='TranslationTextInputViewModel' (HashCode=49954236); target element is 'TranslationTextInput' (Name='InputControl'); target property is 'Translation' (type 'Translation')

Parece que o SelectedEntity é procurado no Viewmodel do UserControl filho, mas a propriedade do ViewModel pai deve ser usada. Como posso resolver isso?

Editar:

    public TranslationTextInputViewModel()
    {
        //EnglishTranslation = tranlsations["en"];
    }

    public string EnglishTranslation
    {
        get
        {
            if (!Translation.TranslationDict.ContainsKey(new CultureInfo("en").LCID))
                Translation.Translations.Add(new TranslationItem() { Text = "", Lcid = new CultureInfo("en").LCID });
            return Translation.TranslationDict[new CultureInfo("en").LCID].Text;
        }
        set
        {
            Translation.TranslationDict[new CultureInfo("en").LCID].Text = value;
        }

    }

    public string SelectedTranslation
    {
        get
        {
            if (!Translation.TranslationDict.ContainsKey(_selectedLanguage))
                Translation.Translations.Add(new TranslationItem() { Text = "", Lcid = _selectedLanguage });
            return Translation.TranslationDict[_selectedLanguage].Text;

        }

        set
        {
            Translation.TranslationDict[_selectedLanguage].Text = value;
        }
    }

    private Translation _translation;

    public Translation Translation
    {
        get
        {
            if (_translation == null)
                _translation = new Translation();
            return _translation; }
        set { _translation = value; }
    }

    private int _selectedLanguage;
    public int SelectedLanguage
    {
        get
        {
            return _selectedLanguage;
        }
    }

    public List<CultureInfo> AvailableLanguages
    {
        get
        {

            return (from x in PqsLocalization.AvailableLanguages where x.Name != "en" select x).ToList();
        }
    }

    public RelayCommand<int> LanguageChanged { get; private set; }


    private void LanguageChangedExecute(int lang)
    {
        _selectedLanguage = lang;
        RaisePropertyChanged("SelectedLanguage");
        RaisePropertyChanged("SelectedTranslation");
    }