JK Rowling has stated that Parseltongue is different from other languages. You can learn to understand it (like Dumbledore) and even mimick it to some degree (like Ron did), but actually knowing Parseltongue comes from one thing and one thing only: being a direct descendant of Salazar Slytherin. More precisely:
This is a weird ability passed down through the Slytherin blood line.
This, coupled with the scenes in the Gaunt house in Half-Blood Prince, would seem to imply that all direct descendants of Salazar Slytherin instinctively know Parseltongue.
But Slytherin lived over a thousand years ago, and simple genealogy will tell you that that means his direct descendants will, by the end of the 20th century, number at the very least several thousand. Probably more. The majority of (non-Muggle-born) British wizardkind would probably be able to trace their lineage back to Salazar Slytherin in a direct line, somehow or other. And yet we seem to hear only of the Gaunts and Voldemort (plus by extension Harry) at the time the books take place.
How is Parseltongue passed on, exactly? Is it perhaps only passed on—at least as an active ability (speaking)—to eldest sons?1
How can a thousand years of genealogy possibly produce only one single, small family of descendants? Has canon/JKR ever addressed this at all?
1 Remember, we never hear Merope uttering a single word of Parseltongue, though obviously she understood it—she may not have been able to actually speak it like her father and brother. She was still able to pass it on to Voldemort, though, which kind of undermines the limiting power of restrictions like claiming only first-born sons inherited the trait—and also precludes any theory that Slytherin descendants had to actively choose to pass on or instruct their children in Parseltongue: Voldemort never knew his mother and was still able to speak it very well.