email_template_design/views/sale_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record id="sale_view_form" model="ir.ui.view">
      <field name="name">sale_view_form</field>
      <field name="model">sale.order</field>
      <field name="inherit_id" ref="sale.view_order_form"/>
      <field name="arch" type="xml">
        <button name="action_quotation_send" position="replace">
          <button name="action_quotation_send" string="Send by Email" type="object" class="oe_highlight" groups="base.group_user" invisible="0"/>
        </button>
      </field>
    </record>
  </data>
</openerp>