Regarding 'interpreting' for congenitally deafblind children: Yes, children who are congenitally deafblind CAN learn to communicate, but all children who are deafblind have unique individual needs and will learn at their own pace and in their own way. As professionals, we must carefully observe and respond to the child to learn what is the best way to go, and then build on his/her personalised communication systems.
In my experience, sharing the interests of the child by offering your hands UNDER his can be a very effective way of developing 'communication', instead of shaping his hands into formal signs that may have no meaning for him yet. When he offers to share his toys with you, follow what his hands are doing and share his exploring. Make your hands available to him, next to his, and allow him to use your hands, rather than you directing his hands. Follow his lead. First, we must acknowledge and 'interpret' the communication efforts of the child before we can expect the child to understand our formal signs. If a child has very few signs, accept HIS signs by imitating them UNDER his hands, then modeling the sign for what you think he is trying to tell you (again with your hands under his). If he has enough functional vision, modeling the signs within his field of vision may be sufficient. If there is no useful vision, model the sign UNDER his hands. (If the child will not initially keep his hands on yours long enough to follow the signs, then making the sign on his arm or back of his hand may be useful). We can then build on HIS communication, towards a more formal system, when the child is ready. This approach will also foster a trusting relationship with the child, as it gives more control to the child, and allows him to learn the power of HIS communication.
Also, all people involved directly with him need to make contact with him in a meaningful and non-threatening way, by always approaching from the front and identifying themselves by offering their hands UNDER his. If there is any functional hearing, saying his name first, then your own name will give him additional information. I hope all this makes sense.
Here in Western Australia, we have an excellent training video called, ''Where do I begin?' Developing communication with children born deafblind', which can be ordered through the Deafblind Specialist Services of SENSES Foundation at: email@example.com
Also, in your own country [France], there are excellent videos available called, 'Emergence of Communication' which have been developed through the Communication Network of Deafblind International. Jacques Souriau would be able to give you info on how to obtain these videos. Jacques' email is: Jacques.firstname.lastname@example.org
deafblind Link has many excellent articles that deal with interacting and communicating with congenitally deafblind children. Authors such as Jan van Dijk, Barbara Miles and Linda Hagood would be a good place to start. deafblind Link's email is: email@example.com