To reproduce:
1.Use the provided xml file in the following article: http://www.telerik.com/help/winforms/richtexteditor-localization.html and change these strings:

"Documents_ParagraphPropertiesDialog_TextAlignment_Center": "Mittig";
"Documents_ParagraphPropertiesDialog_TextAlignment_Justify": "Block";
"Documents_ParagraphPropertiesDialog_TextAlignment_Left": "Links";
"Documents_ParagraphPropertiesDialog_TextAlignment_Right": "Rechts";

2.Afterwards,  specify the RichTextBoxLocalizationProvider.CurrentProvider by loading the modified file. 
3.Run the application, select some paragraph and try to change its alignment. As a result when the ParagraphPropertiesDialog is opened, incorrect alignment text is displayed in the drop-down. If you select a new value a handled ArgumentException is thrown and the paragraph is not aligned at all.

Workaround: use a custom ParagraphPropertiesDialog:
public Form1()
{
    RichTextBoxLocalizationProvider.CurrentProvider = RichTextBoxLocalizationProvider.FromFile(@"..\..\RichTextBoxStrings - Copy.xml");
     
    InitializeComponent();

    this.radRichTextEditor1.RichTextBoxElement.ParagraphPropertiesDialog = new CustomDialog();
}


public partial class CustomDialog : ParagraphPropertiesDialog
{
    public CustomDialog()
    {
        InitializeComponent();
       
        FieldInfo fi = typeof(ParagraphPropertiesDialog).GetField("radButtonOK", 
            BindingFlags.Instance | BindingFlags.NonPublic);
        RadButton btn = fi.GetValue(this) as RadButton;
     
        btn.MouseDown += btn_MouseDown;
    }
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e); 
        if (selectedItem != null)
        {
            ButtonAlignement.Text = selectedItem.Text;
        }
    }

    public RadDropDownButton ButtonAlignement
    {
        get
        { 
            FieldInfo fi = typeof(ParagraphPropertiesDialog).GetField("radDropDownButtonAlignement",
                BindingFlags.Instance | BindingFlags.NonPublic);
            RadDropDownButton radDropDownButtonAlignement = fi.GetValue(this) as RadDropDownButton;
            return radDropDownButtonAlignement;
        }
    }

    RadMenuItem selectedItem = null;
    RadDropDownButton radDropDownButtonAlignement = null;

    private void btn_MouseDown(object sender, MouseEventArgs e)
    {
        foreach (RadMenuItem item in ButtonAlignement.Items)
        {
            if (item.Text == ButtonAlignement.Text)
            {
                selectedItem = item;
                break;
            }
        }
        if (selectedItem != null)
        {
            ButtonAlignement.Text = selectedItem.Tag.ToString();
        }
    }
}