为什么wpf绑定只更新字符串而不更新对象?

在下面的代码示例中,可以将滑块从德语移到英语,并在运行时看到文本块被翻译,但是:

• 只有绑定到 一串 得到更新
• 绑定到 词典 不 获得更新

看来 获取dictionary对象一次 然后就不再更新了。我试过 模式= TWOWAY 但那没有效果。

我必须做什么才能使元素绑定到 物体 通过绑定更新?

观点:

<Window x:Class="TestObjectUpdate234.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:c="clr-namespace:TestObjectUpdate234.Commands"
    Title="Main Window" Height="400" Width="800">
    <StackPanel Margin="10">

        <TextBlock Text="{Binding TranslationEdit}" />
        <TextBlock Text="{Binding Translations[add], Mode=TwoWay}" />

        <StackPanel DockPanel.Dock="Bottom"  Orientation="Horizontal" Margin="0 20 0 0">
            <TextBlock Text="English" Margin="0 0 5 0"/>
            <Slider Name="TheLanguageIndexSlider"

                Minimum="0" 
                Maximum="1" 
                IsSnapToTickEnabled="True"
                Width="100" 
                Margin="5" 
                Value="{Binding LanguageIndex}"
                HorizontalAlignment="Left"/>
            <TextBlock Text="German" Margin="5 0 0 0"/>
        </StackPanel>

    </StackPanel>
</Window>

ViewModel:

using System.Collections.Generic;

namespace TestObjectUpdate234.ViewModels
{
    public class MainViewModel : ViewModelBase
    {

        #region ViewModelProperty: TranslationEdit
        private string _translationEdit;
        public string TranslationEdit
        {
            get
            {
                return _translationEdit;
            }

            set
            {
                _translationEdit = value;
                OnPropertyChanged("TranslationEdit");
            }
        }
        #endregion

        #region ViewModelProperty: Translations
        private Dictionary<string, string> _translations = new Dictionary<string, string>();
        public Dictionary<string, string> Translations
        {
            get
            {
                return _translations;
            }

            set
            {
                _translations = value;
                OnPropertyChanged("Translations");
            }
        }
        #endregion

        #region ViewModelProperty: LanguageIndex
        private int _languageIndex;
        public int LanguageIndex
        {
            get
            {
                return _languageIndex;
            }

            set
            {
                _languageIndex = value;
                OnPropertyChanged("LanguageIndex");
                FillTranslations();
            }
        }
        #endregion

        public MainViewModel()
        {
            _languageIndex = 0; //english
            FillTranslations();

        }

        private void FillTranslations()
        {
            if (_languageIndex == 0)
            {
                TranslationEdit = "Edit";

                Translations.Clear();
                Translations.Add("add", "Add");
            }
            else
            {
                TranslationEdit = "Bearbeiten";

                Translations.Clear();
                Translations.Add("add", "Hinzufügen");
            }

        }


    }
}