A parsel mouth is someone who talks to snakes, it is a rare ability. You can, yes, learn parsel-tongue, but it's not the same. As Ron does in Harry Potter 7 pt 2. he opens the chamber of secrets, you can only do that with parsel-tongue. As I said before it's not the same, you see Ron could yes, speak the language, but he can't really understand it. So unlike Harry or Voldemort, Ron can not however have a conversation with a snake. This proves that you can learn to speak parsel tongue, but you can not really know what it means, in the means of a conversation, for to anyone like Ron, Hermione, and most others it is just a bunch of gibberish.