One of the problems with parseltongue (and any other form of talking to animals in any language) is how to depict the conversations with animals. Most authors fall into an easy trap and over anthropomorphise the animal in question. Mrs. Rowling herself seems to try to walk a line with this. When [Sirius] describes how he survived [Azkaban], he says that his emotions are “less complex” as a dog,1 but Crookshanks seems able to grasp some fairly complex ideas.2 It is important to get right, because if every single animal is actual intelligent, separated from us only by differing desires and a language barrier, then you have denied the uniqueness of the human soul, and its role in making us what we are. On a more practical note, it also has implications on daily life, how can you eat something you converse with? It is for precisely this reason that farmers raising animals for slaughter do not name them.
One possibility is that the animal gets a boost to its ability while the person is talking to it. That is, that parseltongue does not simply grant you knowledge of and the physical ability to pronounce another language, but actually reaches out to the snakes you speak to and grants them a level of intelligence to understand your intent. This avoids many of the problems with the first version, where every animal, or some subset there of (some authors try to distinguish between magical and non-magical animals), is always intelligent.
[Sirius]: </Harrypedia/people/Black/Sirius III/>/ [Azkaban]: /Harrypedia/azkaban/