You are right , the cause of death is never mentionned in a belgian or french " Acte de décès" ( which is what is called "Death certificate" in England) the document that family , or other persons can obtain from the "mairie" ( town hall where the death happened ).
The cause of a death is mentionned in the "Certificat de décès ", which is the document written by the doctor who certifies the death . This "certificat de décès" is given to the "mairie" who sends it to a central office which uses it for national statistics.You will never be able to see this "certificat de décès", the cause of a death is confidential data.
However usually the family knows about the cause of death , asking the family is a way of getting this information.Or if the death was part of History, reading books or the Internet.
