German house owner disputes ownership of a garage in Catalonia with French couple

The French couple is governed by the matrimonial regime of separation of property (having entered an express marriage contract to that effect) and, therefore, each acquired separately and privately the undivided ½ of the building where the garage in dispute is located.
The husband has died, and his heirs must be sued, as must the wife who survived the husband, as this is the only way to constitute the necessary standing to sue.
Given all the international aspects, this was a typical case that we could only deal with with the help of our colleagues in the Eurojuris network.
Our French members informed us that, in France, in order for an inheritance to be effectively passed on to the heirs of the deceased husband, it is first necessary to obtain a notarial declaration of heirs and then a copy of the acceptance of the inheritance by the named heir, or in case the heir had repudiated the inheritance, to identify who is the next last named heir and who had effectively accepted the inheritance. (Logically the wife did not want to provide this information).
As such information is subject to serious difficulties in finding such documents, our colleagues in France found out and we were able to prove who the heir was and so we were able to sue them in court.