Dolphins are not (so far as we know) actively trying to teach us their language. I would be exuberant to be corrected on this, but to my knowledge a dolphin has never put a fish in a human's hands and cried 'kreeeikaaa' to try to teach us the word. Whereas we do that endlessly with the dolphins, which is why they have learned a bit of our language.

I suspect (with no proof) that a human-level intelligence, bent on communicating, would have the intelligence to perform experiments and then structure a learning environment for the other life form to learn its language.