In the code set default template using ctx['default_template_id'] = template_id

Example portal_sale.py code (Send by Email using Template)

    def action_quotation_send(self, cr, uid, ids, context=None):
        '''  Override to use a modified template that includes a portal signup link '''
        action_dict = super(sale_order, self).action_quotation_send(cr, uid, ids, context=context)
        try:
            #template_id = self.pool.get('ir.model.data').get_object_reference(cr, uid, 'portal_sale', 'email_template_edi_sale')[1]
            template_id = ir_model_data.get_object_reference(cr, uid, 'sale', 'email_template_edi_sale')[1]            
            # assume context is still a dict, as prepared by super
            ctx = action_dict['context']
            ctx['default_template_id'] = template_id
            ctx['default_use_template'] = True
        except Exception:
            pass
        return action_dict