Why does Morfin Gaunt mostly speak in Parseltongue?

Question

I've been wondering for some time: why does Morphin Gaunt mostly speak in Parseltongue? It seems very odd, because, well, it makes no sense.

Answer 1

He favors Parseltongue on many points.

Morfin only usually had to speak to his family.

When Morfin is seen, he only spoke to his father and sister, so he could speak only in Parseltongue because they both understood it. Marvolo had to speak English to Bob Ogden because he needed to be understood, so while he may have preferred speaking in Parseltongue, he was forced to speak English.

“Ministry, is it?’ said the older man, looking down at Ogden.

‘Correct!’ said Ogden angrily, dabbing his face. ‘And you, I take it, are Mr Gaunt?”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)}}

Morfin did not have to speak to Ogden, so he was able to converse with his family only in Parseltongue.

Outsiders would not understand their speech.

Since they spoke Parseltongue, a very rare language, the Gaunts were able to speak amongst themselves without many others understanding their conversation. The Gaunts could speak to each other even with a Ministry official right next to them and their conversation would still remain secret.

“Mr Gaunt spoke out of the corner of his mouth to Morfin.

‘Get in the house. Don’t argue.’

This time, ready for it, Harry recognised Parseltongue; even while he could understand what was being said, he distinguished the weird hissing noise that was all Ogden could hear.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)}}

Morfin speaking only Parseltongue ensured that Ogden could not understand what he said.

It was proof he was a descendant of Slytherin.

The Gaunts were proud of their heritage, being pure-blooded descendants of Salazar Slytherin.

“See this?’ he bellowed at Ogden, shaking a heavy gold locket at him, while Merope spluttered and gasped for breath.

‘I see it, I see it!’ said Ogden hastily.

‘Slytherin’s!’ yelled Gaunt. ‘Salazar Slytherin’s! We’re his last living descendants, what do you say to that, eh?”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)}}

The Gaunts’ ability to speak Parseltongue was an indication of their heritage, and as such they would be proud of being born with the ability to speak it.

“You can speak Parseltongue, Harry,’ said Dumbledore calmly, ‘because Lord Voldemort – who is the last remaining descendant of Salazar Slytherin – can speak Parseltongue.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 18 (Dobby’s Reward)}}

Though in Dumbledore’s memory they have a visitor, so would have a reason to want to speak secretly, it is likely they spoke Parseltongue as their primary language at home when they were alone as well, due to its representation of their heritage.

Morfin could speak English, though.

However, Morfin was capable of speaking English should he choose to do so. When Voldemort went to the Gaunt house, he yelled in English until Voldemort responded in Parseltongue.

“YOU!’ he bellowed. ‘YOU!’

And he hurtled drunkenly at Riddle, wand and knife held aloft.

‘Stop.’

Riddle spoke in Parseltongue. The man skidded into the table, sending mouldy pots crashing to the floor. He stared at Riddle. There was a long silence while they contemplated each other. The man broke it.

‘You speak it?’”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)}}

He confessed to killing the Riddles to the Ministry of Magic, which he almost certainly would have needed to do in English for them to understand him.

“So the Ministry called upon Morfin. They did not need to question him, to use Veritaserum or Legilimency. He admitted to the murder on the spot, giving details only the murderer could know. He was proud, he said, to have killed the Muggles, had been awaiting his chance all these years. He handed over his wand, which was proved at once to have been used to kill the Riddles. And he permitted himself to be led off to Azkaban without a fight. All that disturbed him was the fact that his father’s ring had disappeared. “He’ll kill me for losing it,” he told his captors, over and over again. “He’ll kill me for losing his ring.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)}}

He also kept telling them that his father would kill him for losing his ring, which he would also presumably have to do in English for it to be understood. Additionally, this would not have been planted in his head by Voldemort. This would not benefit Voldemort so he would have no reason to plant that thought. Morfin’s concern at his father’s reaction to his losing the ring would be entirely his own thoughts. It would not have been in any way ‘helped’ in its translation by Voldemort having put it in his mind. Therefore, he could speak English as well, though he favored Parseltongue.

Answer 2

Morfin likely used Parseltongue as a sign of superiority.

Parseltongue further separates them from Muggles. It was a Wizard-only language especially linked to Salazar Slytherin. The Gaunts, with their direct decent from Slytherin, and their arrogance and disdain for Muggles would likely have considered Parseltongue as much an heirloom and mark of their superiority as the ring and locket.

"See this? See this? Know what it is? Know where it came from? Centuries it's been in our family, that's how far back we go, and pure-blood all the way!" - Marvolo Gaunt to Ogden, HBP

...

"See this?" he bellowed at Ogden, shaking a heavy gold locket at him, while Merope spluttered and gasped for breath.

"I see it, I see it!" said Ogden hastily.

"Slytherin's!" yelled Gaunt. "Salazar Slytherin's! We're his last living descendants, what do you say to that, eh!" - HBP

Answer 3

There are two main reasons why Morfin would communicate mostly in Parseltongue.

First off, the entire Gaunt family seems to have communicated largely in Parseltongue. We see examples of that when Dumbledore and Harry dive into Ogden's memory:

Mr. Gaunt spoke out of the corner of his mouth to Morfin. "Get in the house. Don’t argue." This time, ready for it, Harry recognized Parseltongue; even while he could understand what was being said, he distinguished the weird hissing noise that was all Ogden could hear.

The jingling, clopping noises were growing louder and louder. Morfin made to get out of his armchair. "Keep your seat," said his father warningly, in Parseltongue.

"'Darling,'" whispered Morfin in Parseltongue, looking at his sister. "'Darling,' he called her. So he wouldn’t have you anyway."
Merope was so white Harry felt sure she was going to faint. "What’s that?" said Gaunt sharply, also in Parseltongue, looking from his son to his daughter. "What did you say, Morfin?"

We can see from these quotes that the family communicated a lot in Parseltongue, so it would make sense for Morfin to speak it.

The second example is that Morfin is, well, probably not entirely sane. We see several examples of this at the same time:

"Mr. Gaunt, please!" said Ogden in a shocked voice, as Merope, who had already picked up the pot, flushed blotchily scarlet, lost her grip on the pot again, drew her wand shakily from her pocket, pointed it at the pot, and muttered a hasty, inaudible spell that caused the pot to shoot across the floor away from her, hit the opposite wall, and crack in two.
Morfin let out a mad cackle of laughter. Gaunt screamed,
"Mend it, you pointless lump, mend it!"
_{(emphasis added)}

"Don’t you go talking to us as if we’re dirt on your shoes! Generations of purebloods, wizards all — more than you can say, I don’t doubt!"
And he spat on the floor at Ogden’s feet. Morfin cackled again.
Merope, huddled beside the window, her head bowed and her face hidden by her lank hair, said nothing.
_{(emphasis added)}

"Good lord, you’re right!" said the man’s voice. "That’ll be the son, I told you he’s not right in the head. Don’t look at it, Cecilia, darling."
The jingling and clopping sounds were now growing fainter again.
_{All quotes from Harry Potter and the Half-Blood Prince, chapter 10: "The House of Gaunt"}

So, these two things together - that his family spoke Parseltongue, perhaps as their main means of communication, and that he may not have been entirely sane - probably explain why Morfin communicated largely in Parseltongue.