For example for Input Method Editors like Marathi Indic Input, part of MS Indic Language input tool, downloadable from here: http://www.bhashaindia.com/ilit/Marathi.aspx. Workaround: RadRichTextBox currently exposes customization point for implementing custom IMEs, see Custom IME Support: http://docs.telerik.com/devtools/wpf/controls/radrichtextbox/ime-support#custom-ime-support