Harry can no longer speak it
Lord Voldemort and Nagini had a special relationship
As a Parselmouth, (i.e. able to talk to snakes) Voldemort could communicate with her, and the pair often sent each other messages, even suggesting a telepathic connection at times.
When Harry and Hermione are on the first floor of Bathilda's house, Nagini (inside the corpse of Bathilda) tells Harry to "Come!" from the next room in Parseltongue. In reaction, Hermione jumps and clutches Harry's arm, and the two of them obey the command.
Edit: after writing this I realized Harry DOES talk to Nagini when she's inside of Bathilda, he just doesn't realize it.
Harry can see through Voldemort's eyes from time to time because Harry has one of Voldemort's horcruxes in his head. Harry now has the ability to see through Nagini's eyes. Apparently, Harry and Nagini are now joined via horcruxes. Nagini has a portion of Voldemort's soul inside her as well.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanently. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.
Nagini is loyal to Voldemort because he can speak with snakes. As a Maledictus, there probably isn't much time after the events in The Crimes of Grindelwald before Nagini falls to her blood curse, becoming a snake forever.
There was a rumour that Nagini was the snake Harry released from the zoo in Harry Potter and the Philosopher's Stone. This was proven false, as Nagini is some type of viper while the snake in the zoo was a boa constrictor. Furthermore, the snake was given a male voice in the film, while Nagini is female.
Rowling previously revealed that Nagini is a Maledictus, which means she carries a blood curse that will doom her to transform permanently into a beast. In a film extra called "Credence, Nagini and the Circus Arcanus," available on the "Crimes of Grindelwald" Blu-ray, Rowling said she "knew all along about Nagini."
Voldemort orders Nagini to kill Snape, and the great snake bites Snape's neck, mortally wounding him.
Dumbledore does not use any language to make one think that Nagini is anything more than a snake. But this could be despite his knowledge of Nagini's past because he has to factor his grand plans into everything he tells (or doesn't tell) Harry.
It wasn't just inanimate objects that could become Horcruxes. Lord Voldemort's beloved snake companion, Nagini, was also transformed into one, and Dumbledore seemed to know it, ominously predicting that a day would come where Voldemort would 'fear for his snake'.
The wand will not perform its best for Voldemort until Snape is dead. So saying, Voldemort waves his pet snake, Nagini, in her nest of protective spells, into Snape.
Deathly HallowsEdit
Nagini, Voldemort's snake, then eats Professor Burbage's dead body. A story about her "resignation" from Hogwarts appears in the Prophet, but members of the Order of the Phoenix discount it, as they are unable to locate her anywhere after that.
In the movies, Nagini was the one who killed Snape in the boathouse. The Maledictus meets her end when Neville Longbottom finds Gryffindor's sword and, in an attempt to destroy the final Horcrux, cuts off her head.
Once he learned that his horcruxes were being hunted, he knew Nagini was at risk. He couldn't know how his enemies had been able to find his other horcruxes, but he had to suspect at the least the possibility that it was by some magical means unknown to him, perhaps devised by Dumbledore.
Troubled, Lord Voldemort spoke to Nagini in Parseltongue, telling her that their secret had been discovered and warning her to never leave his sight, before Disapparating to a cave to seek out the locket of Salazar Slytherin.
7 horcruxes were diary, ring, locket, cup, tiara, Nagini and Harry. They were destroyed by Harry, Dumbledore, Ron, Hermione, Crabbe, Neville and Voldemort respectively. Amongst them, only Neville and Crabbe didn't know what Horcrux actually were.
But now we know Nagini was no simple supervillainous pet, but a human woman cursed with the “Maledictus” blood curse, which, according to Rowling, is a female-only inherited condition which grants the affected individual the ability to transform into an animal at will, until one day, the transformation becomes ...
As an unqualified wizard who didn't finish his education, Hagrid was not technically allowed to perform magic.
The second film in the series, written by Harry Potter architect, J.K. Rowling, revealed in a trailer that fans had been on the right trail. Nagini, it turns out, was actually a Maledictus—a witch with a blood curse that turns her permanently into a beast (in her case, a giant snake).
Nagini is the last Horcrux that Voldemort created and the last one to be destroyed. During the final battle of Hogwarts, Neville pulls the sword of Godric Gryffindor from the Sorting Hat and beheads Nagini, destroying the last Horcrux.
She was dieting and didn't want greasy food.
Funnily enough, the most famous Lestrange is one only by marriage, and despite really adorable Instagram photos that make us wish it so, there is no blood relation between Leta and Bellatrix whatsoever. At most, Leta and Rodolphus Lestrange (Bellatrix's husband) are very distant cousins.
Voldemort has no wish to know anything about anyone outside of himself. It simply wasn't important to him as it didn't give him any power or knowledge. However, it may have helped him use her as a Horcrux possibly because Nagini being formerly human may have helped her be strong enough to become a Horcrux.