Can a powerful model become a fantastic autocomplete for dolphins ? Sure. Someday soon that's very likely to happen. But that alone would tell us almost nothing of what dolphin dialogue means. To understand their language we need shared experiences, shared emotions, common internal worlds. Observation of dolphin-dolphin interaction would help but to a limited degree. It would help if the dolphins are also interested…
Why? With modern AI there exists unsupervised learning for translation where you don't have to explicitly make translation pairs between the 2 languages. It seems possible to eventually create a way to translate without having to have a teaching process for individual words like you describe.