Article 1342-10 of the French Civil Code
A debtor of several debts may indicate, when paying, which debt he intends to discharge. Failing indication by the debtor, the imputation takes place as follows: first on the debts due; among these, on the debts which the debtor had the most interest in discharging. In the event of equal interest, the charge is made against the oldest; all other things being equal, it is made proportionally.