My undergraduate psych professors were primarily dolphin and manatee researchers, so I have a lot of hearsay, but have done a lot of reading about them, too.

To my knowledge, psychologists primarily have been focusing on capturing the calls of wild dolphins and running frequency analyses to characterize the various noises they hear. There are clicks and click trains, whistles, crazy broad frequency burps, and dolphins seem to have some special skull features that can create a loud crack. All I've really seen major evidence for is that there are characteristic whistles that each individual tends to produce, and many believe these are names or identifiers, but really this is far from certain. These 'signature' whistles are heard when dolphins approach other dolphins, perhaps as a 'Hello' or 'It's me,' but they are also heard in a wide variety of other contexts.

In my opinion, the sounds they make are very diverse and trying to associate these sounds with context, including social contexts, is a very daunting task. I personally believe the complexity of the dolphin acoustics and auditory system is wildly understated, and there are nuances of the calls that time-frequency analyses won't pick up.

As for your question, I have heard that indeed there are 'dialects' across world populations, in so far that the stereotypical calls we hear under certain circumstances may have varying features across locations. But these calls also vary across context, pods, and individuals, so your question isn't super well posed. If you were to ask if dolphins would not be able to recognize any other dolphin calls, perhaps from dolphins a world over, I would likely disagree with that. However, if calls were taken out of context from a different pod halfway across the world, I doubt another dolphin would derive much meaning from them. The question is what were the factors influencing where the meaning of their calls come from? Is it some sort of cultural factor, or is it idiosyncratic to discrete social milieu?

In essence, there is still much debate about the presence of what we think of language in dolphins. We would need to answer that question first to answer your question as you intended to ask it. Though, I'm most likely ignorant of any possible research answering your questions directly, and this is all my intuition gleaned from the brief exposure I had during undergrad.