Like some others, it's occurred to me to use computers to catalog all the sounds a dolphin makes and then create an artificial language based on those sounds. A computer program could then be written that translates back and forth. Young dolphins could be trained in that language by scientists while still being trained in regular dolphin communication by older dolphins. We would then have a group of dolphin ambassadors.
One problem is that the language you develop will probably only be appropriate to that dolphin. Humans have hundreds of languages. There is no reason to believe all dolphins use the same one. If they do share complex information through vocal communication, it's likely that dolphin communities around the world have their own language families, dialects, and even some language isolates.
Dolphin communication probably also consists of body language, so translating only their sounds wouldn't necessarily give you any useful information.
In other words, this is a much, much more difficult problem than people may assume.