There are international standards for the work of sign language translation. They rotate every 20-30 minutes to rest their hands and body, and have to be present at the event to follow the conversation. Usually they get handsout with briefings of the subjets that are going to be translated.
I am not watching Cristina's speech on TV right now. I think it is good to promote the rights of the disabled people having official speeches translated to sign language.
I dont know the specific number of deaf people in Argentina, but I can tell you that 10% of the global population is disabled, that makes the group of disabled people to be the biggest minority of the world: 650 million people. There is an International Convention to Protect and Promote the Rights of the Disabled People and yes, Argentina and the USA, among other countries, signed it some years ago.