invoiceninja/app/lang/fr/fields.php

<?php 

return array(

  // client
  'organization' => 'Entreprise',
  'name' => 'Nom',
  'website' => 'Site web',
  'work_phone' => 'Téléphone',
  'address' => 'Adresse',
  'address1' => 'Rue',
  'address2' => 'Appt/Batîment',
  'city' => 'Ville',
  'state' => 'Région/Département',
  'postal_code' => 'Code Postal',
  'country_id' => 'Pays',
  'contacts' => 'Informations de contact', //if you speak about contact details
  'first_name' => 'Prénom',
  'last_name' => 'Nom',
  'phone' => 'Téléphone',
  'email' => 'Email',
  'additional_info' => 'Informations complémentaires',
  'payment_terms' => 'Conditions de paiement',
  'currency_id' => 'Devise',
  'size_id' => 'Taille',
  'industry_id' => 'Secteur', // literal translation : Industrie
  'private_notes' => 'Note personnelle',

  // invoice
  'invoice' => 'Facture',
  'client' => 'Client',
  'invoice_date' => 'Date de la facture',
  'due_date' => 'Date d\'échéance',
  'invoice_number' => 'Numéro de facture',
  'invoice_number_short' => 'Facture #',
  'po_number' => 'Numéro du bon de commande',
  'po_number_short' => 'Bon de commande #',
  'frequency_id' => 'Fréquence', //litteral translation : Combien de fois 
  'discount' => 'Remise', //can be "rabais" or "réduction"
  'taxes' => 'Taxes',
  'tax' => 'Taxe',
  'item' => 'Ligne', //I'm not sure, I need the context : screenshot ? 
  'description' => 'Description',
  'unit_cost' => 'Coût à l\'unité',
  'quantity' => 'Quantité',
  'line_total' => 'Total',
  'subtotal' => 'Total',
  'paid_to_date' => 'Versé à ce jour',//this one is not very used in France
  'balance_due' => 'Montant total',//can be "Montant à verser" or "Somme totale"
  'invoice_design_id' => 'Design', //if you speak about invoice's design -> "Modèle"
  'terms' => 'Conditions',
  'your_invoice' => 'Votre Facture',
  
);