HTTYD both books and movie universe, is post apocalyptic, I SWEAR.
Okay so I've been reading the books recently, and ever since we've seen the Romans in book 3, I have had a sneaking suspicion that at least the book universe is post apocalyptic.
Exhibit 1: The Romans, the time of the Roman Empire and the time of the Norse stuff were at LEAST 100 years apart. How are there true blooded, Latin speaking, empire building Romans in the books?
Source: How To Speak Dragonese
Exhibit 2: HICCUP SPEAKS FRENCH. He's done it multiple times and, and not only did he do it, but so did Alvin (rather poorly but still so) in book 8. (This is impressive because Hiccup officially seems to be fluent in four separate languages, Nordic, Latin, French, and Dragonese ((The language the dragons speak in the books)))
Exhibit 3: They nearly go to AMERICA. NOT THE NEW WORLD OR THE WEST. AMERICA. It's mentioned in book 4 where Hiccup and Camicazi have to get the potato, which is specifically only from America in the books, confirmed by Norbert the Nutjob. But then in book 7 Norbert captures Hiccup, Camicazi, and Fishlegs, and they nearly get to America, it's in sight, it was right there. Sure the existence of America isn't a stretch, but the name. The name. It implies that all of the whole America "discovery" thing already happened. And sure, the Norse made it to America way before Britain did, but it's the name that matters.
Source: How To Ride a Dragon's Storm
Sure, these could all just be part of Cressida Cowell not really caring about historical accuracy, but that's a lame answer. Besides, the movie universe has this too.
On multiple occasions, Ruffnut and Tuffnut have referenced and/or spoken languages from after the Norse' time. Usually Spanish.
(Listen I have the books right here with me yeah, but I'm not gonna go look through every single episode of the HTTYD shows just to find him only doing those things.)
He also mentions French once, and Snotlout has this funny joke. They can be excused because they're meant to be comic relief, not really supposed to be taken seriously all too much. However, in Season 6 of Race to the Edge, in Episode 10, Darkest Night, this exchange happens.
Snotlout: What are you talking about? Hookfang's scales change every time he molts.
Fishlegs: It's a metaphor. A figure of speech. Not literal.
Snotlout: Okay, keep your witch trickery to yourself.
Fishlegs: It's not trickery! It's English!
FISHLEGS DIRECTLY SAYS ENGLISH. And he says it in reference to the language they speak, when it should be Scandinavian. This could just be a joke, because it happens during a joke thing, but still! It's direct evidence that the Vikings in How to Train Your Dragon speak English, a language that shouldn't exist until far after these characters' deaths, case CLOSED.
...But what caused the apocalypse? There's gotta be a reason, right? Well it's obvious. It was the dragons.
Initially in HTTYD 1 the Vikings are at war with dragons because of just how many they've killed. But by then, there are already people who are at peace with dragons, why isn't Berk? Easy, they're stubborn, it's an occupational hazard. They don't give up. Why? Because they know just how much destruction they caused for the earth. So much so that they wiped out every single structure and remnant of the old world. So much so that new generations only have small glimpses of what the world once was. You can't just forget that kind of thing.
But by now, if you know the biggest difference the books and movie universe, you'd know that everyone is at peace with dragons during the main events of the story. Why? That I don't know. I needed to get this out and still don't have that question answered. Anyway thanks for coming to my rant.