The specific elemental languages are said to be dialects of Primordial. The rules aren’t entirely clear on what that means - can a creature that speaks Auran but not Aquan still communicate effectively with a creature that speaks Aquan but not Auran? Can a character who speaks Primordial effectively communicate with creatures that speak the element-specific dialects? This is all just kind of left up to the DM.
Personally, I rule that the elemental dialects are not pronounceable by non-native speakers. Ignan, for example, involves sounds like the cracks, pops, and hisses of a burning fire, and can’t be reproduced by the human(oid) tongue, but a dedicated scholar can learn to understand them. “Primordial” is a sort of constructed language that is pronounceable by most humanoids. Any creature that can speak one of the 5 languages can understand the other 4, but they can only speak in the one they know. So which one a creature speaks is largely descriptive flavor.