Disclaimer: I do not own Harry Potter...
"How did Ron Weasley open the Chamber of Secrets?" you may have wondered once or twice while reading or watching Harry Potter. "He wasn't a parselmouth, was he?"
Well, I have brought you a theory you can think of for a while, a theory that might explain some things. Keep in mind that this is only a theory, and read ahead.
Parseltongue is a magical ability as we all know, and wizards and witches obviously have magic. It makes sense to me that thay can learn parseltongue like any other (complicated) magical language, like mermish, or gobbledegook. That what happened with Ron. He'd listened to Harry speak it, and managed to copy him. Dumbledore also learnt parseltongue somehow, but did not speak it, possibly because parselmouths were seen as "Dark", and he had a reputation to keep and a war to win.
But as I see it, people who learnt parseltongue from the ones born with the ability couldn't actually communicate with snakes. Why? They lack the magic that comes with innate parseltongue, and that magic cannot be learnt. Just like you cannot learn to grow a sixth finger.
"Why would that parselmagic matter? We are talking about talking to snakes, and they usually haven't got any magic", you wonder, and that's an excellent question. Ten points to Hufflepuff. You're right, and we'll get to the importance of parselmagic in a moment.
Non-magical snakes don't seem to need magic to be understood by a parselmouth, but parselmouths need magic to be understood by the snakes. Because the thing is: snakes do not really have a language. They do not hiss with each other about the weather or delicious puppies. They can scream, and that's about it. They aren't stupid, however. They can learn.
"Well, how in Merlin's name could Harry and Voldemort converse with snakes, if they have no language?" And here comes the second part of my theory: magic. Magical language. Parselmouths' natural parselmagic influences the snake's brain when a parselmouth hisses to it. The parselmagic makes the snake smarter, capable of snake-speech. No spell or enchantment could do that like the subtle parselmagic. Of course, it takes some time and lot of conversation that the snake becomes intelligent enough to understand and talk in complicated sentences.
It makes sense when you think about the boa in the zoo. He seemed intelligent enough, and polite. I'd bet that Harry wasn't the first parselmouth to talk to him over the years, as boa constrictors can live up to 25 years. Some of the parselmouths that the boa met were probably spanish, because how else could a snake learn to be polite and to speak spanish in a zoo?
It also makes sense with Nagini. She had above avarage intelligence, for a snake, before meeting Voldemort. That intelligence came from being a human for part of her live, if the hints in Fantastic beasts were to be believed. Then with Voldemort, she was continuously talked to in parseltongue, therefore getting the human-like intelligence that she was portrayed with.
Wondered why the basilisk in the Chamber did not talk at all? It was under an enchantment, plus had been in there for thousand years. It had forgotten much of the speech it had been taught over the years.
We muggles do not have magic, so we might learn a few words of parseltongue, mermish, or gobbledegook if we're lucky. Talking to passing snakes or goblins in their own language will be out of the question, then. You can of course try, but they'll probably just be very bewildered, and your neighbors will think you're crazy. I advise to not try. Especially if the snakes are wild, poisonous ones.
Non-parselmouth witches and wizards may hiss to each other in badly pronounced parseltongue and possibly understand each other, but snake charmers they'll never be.
Answer to the question about Ron Weasley and why the Chamber opened even though he wasn't a born parselmouth: he opened the Chamber by learning how to hiss 'open' correctly. The entrance was charmed to only recognise 'open' as a password, not to recognise when the speaker actually had parselmagic inside them. That was possibly because Salazar Slytherin wanted his wife and non-parselmouth kids to be able get to his super secret chamber or something. Who knows.
There is my theory. Thank you for taking out time to read my ramblings. You may borrow my theory in your fics if you want, but send a PM to tell me which story, so I may read it too. And add my name in your Author's Notes as the creator of this theory.
Apologies if I unknowingly copied someone's already existing theory. I can solemny swear I was not aware of it.