For a moment during that scene, I expected that there would be some second revelation that Nidhogg was involved in Ratatoskr's death. He seems characteristically malicious, especially in the scene where he pressures Hraesvelgr for half of his power. It could still be revealed that Nidhogg was inherently evil and was actually involved; perhaps he is the one who tempted Thordan in the first place. Right now, however, we've only been told that Nidhogg was skeptical of mankind to begin with and that his rage at their betrayal consumed him entirely. There's some reason to doubt it, but nothing to contradict it. Who's to say it wasn't the Ascians who put Thordan on that train of thought?
At least we know that it probably wasn't the Allagans >.>
The other languages are similar, so what is required is the correct interpretation.
Haldrath is nothing like the image of his father that we've been given. Thordan wanted power bad enough to end a two-century peace and bear the breaking of a pact. Thordan and some knights were slain in Nidhogg's resulting fury, but Haldrath and the remaining knights were able to overcome him with their newfound power.
Haldrath is looking back, ashamed, wondering: If Thordan understood the cost of Ishgard's power and prosperity, would he still have done it?
Sylvetrel de Dzemael doesn't share his wistfulness; he just sees that Nidhogg has been killed, Hraesvelgr won't challenge them, any other great wyrms have fled Eorzea, and they possess not only Ratatoskr's power in their blood, but both of Nidhogg's eyes. He wants to see Ishgard rise to power with Haldrath at its head as he enforces, finally, peace. To him, they've traded honor and their treaty with the dragons for strength, independence, and prosperity, and it's time to move on.
Haldrath, however, can't bring himself to do it. Unlike the others, he thinks the sacrifices were too much and that their honor was traded in vain. As his penance for partaking of Ratatoskr's eyes, he forsakes the throne and vows to use the power he has stolen to protect his people from the rage they have stoked in the dragons (as Nidhogg's followers remain).
Flavien de Fortemps joins Sylvetrel, claiming that Haldrath doesn't have the right to abdicate because their entire culture will collapse as a flock without a shepherd. Haldrath retorts that an intact structure of nobility with his trusted knights at its head should be enough, and, that if it's not, they should choose a new king from amongst it while he atones for his father's sins until his death.
Dzemael and Fortemps can't accept that. They say that they've sacrificed too much and fought too desperately to lose Ishgard's culture, and that the best course of action is to (perhaps "as their king (sort of) decreed") divide the leadership amongst the remaining knights as the foundation of the new nobility.
Haillenarte, and Durendaire concur, but (if I remember correctly) four knights are dead, and the remaining four claim that (because Thordan is dead and Haldrath refuses the throne) the Knights Twelve have no purpose and are thus disbanded. The "Brawny Knight," for instance, says that without a duty as a knight, he will accept a duty as a son and follow in his father's footsteps of owning a tavern / inn.
With only the four still there, Sylvetrel says that whoever is king has become irrelevant and that they can elect one who is worthy when one who is worthy arises. All that matters, he claims, is that their culture remain intact, and that to do that will require them to keep these events secret and forge a new history "for the good of Isghard."
Make more sense?