From a machine learning perspective this doesn't seem like such a daunting task. We have pretty good speech recognition for human language (plus acceptable speech synthesis). Assuming dolphins have a smaller vocabulary than humans, speech recognition of dolphins should strictly speaking be a simpler problem (although it would be significantly harder to record the training data).
Dophin 1, "Hey I bet we can get that thing to say 'bite me' when it greets someone!"
Dolphin 2, "You think? Ok, every time we meet in front of them you say 'bite me' and i'll say 'wheree?'"