title: Translate string from sxw file
body: <p>I would like to translate raw strings I added in the sxw <code>Invoice</code> report.</p>
  
  <p>Former string from this report such as <code>Taxes</code> are translated, but I don't know how to translate 'raw strings' associated to custom field I added to the model <code>Invoice</code>...</p>
  
  <p>Even some native model field label in the sxw report, such as <code>account.invoice.date_due</code> are not translated anymore. I don't understand why.</p>
  
  <p>Thank you for your help</p>
  
  <p>Cheers</p>