There are some special characters in the Polish alphabet, which are inserted with key combinations - AltGr + E should insert "ę", Alt Gr + L should insert "ł". These combinations are overridden in RichTextBox because are triggering default commands in Silverlight.
Workaround: Attach to the PreviewEditorKeyDown event:
this.radRichTextBox.PreviewEditorKeyDown += (sender, args) =>
{
if (Keyboard.Modifiers.HasFlag(ModifierKeys.Alt) && Keyboard.Modifiers.HasFlag(ModifierKeys.Control) && args.Key == Key.E)
{
args.SuppressDefaultAction = true;
args.OriginalArgs.Handled = true;
this.radRichTextBox.Insert("€");
}
};