Hello YannCopp,

Yes I finally found a solution :

In python, I inherit sale.order :

class Sale_order(models.Model):
    _inherit = 'sale.order'
    tabTaxes = fields.Many2many('account.tax', compute='_compute_my_taxe', string='tabTaxes')
    @api.multi
    def _compute_my_taxe(self):
        maliste = []
        for obj in self.order_line:
            for taxe in obj.tax_id:
                maliste.append(taxe.id)
        self.tabTaxes = list(maliste)
    @api.one
    def get_amount(self, taxeNom):
        total = 0
        for ligne in self.order_line:
            for taxe in ligne.tax_id:
                if (taxeNom == taxe.name):
                    try:
                        total += ligne.price_reduce * (taxe.amount / 100.)
                    except:
                        total += 0
        return "{0:.2f}".format(total)

•   _compute_my_taxe : to get list of my taxes.

• get_amount : to get amount of that taxes.

Then in my report xml :

<t t-foreach="doc.tabTaxes" t-as="v">
  <tr>
    <td>
      <span t-esc="v.name"/>
    </td>
    <td>
      <span t-esc="doc.get_amount(v.name)[0]"/>
    </td>
  </tr>
</t>

But I think I'll try using dictionnaries, like I did here : http://stackoverflow.com/questions/34122439/odooqwebdictionary-foreach-print-key-and-value/35655404

And if I wouldn't able to connect sale.order.line with account_tax, I'll use request SQL :

SELECT *
FROM sale_order_line a
 ,account_tax_sale_order_line_rel b
 ,account_tax c
WHERE a.id = b.sale_order_line_id
 AND b.account_tax_id = c.id

And I'll adapt my request with :

@api.one
def get_taxes(self, myparameter):
	self._cr.execute("""<MYREQUEST_WITH_PARAMETER>=%s""", (myparameter,))
	_res = self._cr.dictfetchall()
	if not _res: #To control if it's not empty
		return #???
	return _res[0]

• myparameter : it will be my ID of sale.order  

• MYREQUEST_WITH_PARAMETER : it will be my request

• _res[0] : it will return a dictionary

But I don't know yet how to handle empty result (if not _res).