Using code behind or localization file to change the text of the Design, Preview or HTML buttons break the functionality that enables the end-user to change the modes.

The following snippet can be used as a temporary workaround.

<style type="text/css">
    /* Fixes the visual glitch */
    .reModes a {
        width:auto !important;
    }
</style>

<telerik:RadEditor runat="server" ID="RadEditor1" RenderMode="Lightweight" OnClientLoad="OnClientLoad">
</telerik:RadEditor>

<script type="text/javascript">
    function OnClientLoad(editor, args) {
        var $ = $telerik.$;
         
        if (editor.get_renderMode() != Telerik.Web.UI.RenderMode.Lite) return;

        var designAttr = editor.get_localization()["RadEditorDesignMode".toLowerCase()];
        var htmlAttr = editor.get_localization()["RadEditorHtmlMode".toLowerCase()];
        var previewAttr = editor.get_localization()["RadEditorPreviewMode".toLowerCase()];
        
        $("a[title=\"" + designAttr + "\"]").attr("title", "Design");
        $("a[title=\"" + htmlAttr + "\"]").attr("title", "HTML");
        $("a[title=\"" + previewAttr + "\"]").attr("title", "Preview");
    }
</script>