Workaround:
private void radChat1_ItemFormatting(object sender, ChatItemElementEventArgs e)
{
ChatTimeSeparatorItemElement separatorElement = e.ItemElement as ChatTimeSeparatorItemElement;
if (separatorElement != null)
{
if (separatorElement.Text == "TODAY")
{
separatorElement.Text = "TODAY_EN";
}
else if (separatorElement.Text == "YESTERDAY")
{
separatorElement.Text = "YESTERDAY_EN";
}
}
}