This question already has an answer here:
- Why Didn't Dumbledore Hear the Basilisk? 12 answers
Parseltongue could only be spoken by certain wizards such as Gaunts, Voldemort, Harry (temporarily) and Slytherin. Why couldn't it taught like French or Latin? Why wasn't it a subject in Hogwarts? Ron could mimic it to open Chamber of Secrets. During the 6th book Harry visits Gaunt's house with Dumbledore in memory where he sees them talking in parseltongue. Could Dumbledore understand them and how?