Is Nom de la Societe your lookup field, and it populates the Pays field?
What you are saying is that you have assigned a default value to the Pays field and what you are looking for is that if the Lookup doesn't return a value then you want it to populate the default value "Polynese Francaise"?
If this is your request, then I believe you will need to use a different approach. Have the lookup populate the Pays field "hide this field (Always) Set Save the Data.
Create a new Pays field (Variable could be PaysDisplay). Put an IF formula in the field, =IF(PaysLookup Field="","Polynese Francaise",PaysLookup Field)