After reading the Pottermore article on "Ilvermorny School of Witchcraft and Wizardry," by J.K. Rowling, it's unclear to me whether Isolt Sayre was a Parselmouth.
Based on what I remember from reading the Harry Potter books, I would define the term "Parselmouth" as follows:
- A Parselmouth is a person who exhibits the magical ability to understand snakes, hearing what they "say" in verbal form, and who also is able to communicate verbally with snakes by speaking the language Parseltongue. A Parselmouth can speak and understand Parseltongue without being taught. Based on Harry's experience, Parselmouths don't necessarily realize when they are speaking Parseltongue: it happens automatically when they talk to snakes. And similarly, the speech of snakes, and other Parselmouths speaking Parseltongue, apparently sounds to Parselmouths like whatever human language they would expect to hear.
We additionally know that being a Parselmouth is extremely rare and strongly associated with being a descendant of Slytherin.
According to the Ilvermorny article, Isolt Sayre was a descendant of Slytherin who "claimed to understand" a Horned Serpent she encountered in North America—something that surprised another North American magical creature, the Pukwudgie William.
In addition, one of her daughters was rumored to have the ability to speak Parseltongue:
There was a rumour, never confirmed by her family, that, unlike her sister Martha, Rionach was born with the ability to speak Parseltongue and that she was determined not to pass on Slytherin ancestry into the next generation
But on the other hand, the article also says that "Isolt could not speak the language [Parseltongue]". This is given as an explanation for why Isolt is unable to activate Slytherin's wand, which had been taught to "sleep" when given a special command in Parseltongue. I initially thought that this quote clearly established that Isolt was not a Parselmouth of any kind.
Unfortunately, after thinking it over a bit, I became less sure, because I started to think that this statement maybe could be interpreted in context as meaning that Isolt, unlike her evil aunt Gormlaith, could not intentionally produce Parseltongue; i.e. outside of the context of speaking directly to snakes. We see in the books that Harry, who seems to have acquired all the innate abilities of a Parselmouth, initially cannot speak in Parseltongue except for when he is talking to a snake: to get into the Chamber of Secrets, he has to practice a bit by imagining that a scratched image of a snake is a real snake that he is talking to. There are no indications that Isolt ever tried anything like this: in fact, we are told that she didn't want to activate the wand anyway ("she no longer wanted to touch the wand that was the last relic of her unhappy childhood").
So based just on the information in these quotes, the situation seems a bit ambiguous to me. Does anybody know of any more information that would resolve this ambiguity one way or another? There might be something else in the article that I've missed, or there might be more information in other material written by J.K.R. that would indicate whether Isolt Sayre was born with the ability to speak to snakes using Parseltongue.