Siegfried Ramler travelled from Honolulu to London last month, a little short of his 90th birthday, to give a talk about human dignity and the Nuremberg trials, at which he worked as an interpreter. Coincidentally, he arrived as the Conservative party announced an ill-considered and petulant threat to withdraw from the European Convention of Human Rights, aligning itself with the ideas of Vladimir Putin. The convention was adopted shortly after Ramler completed his Nuremburg assignment, reflecting the “enthronement of human rights” that Winston Churchill had called for in October 1942.

Sig, as he likes to be known, has a wry sense of humour and a gentle German accent. A packed audience at the Army & Navy Club on Pall Mall listened in a state of thrall as he described the experience of sharing a small interrogation room with the likes of Hermann Göring and Hans Frank, men whose acts prompted European states to embrace the revolutionary idea of individual human rights. Organised by the International Association of Conference Interpreters (AIIC), the event celebrated the birth of simultaneous interpretation at the Nuremberg trials, a novel development that took years off the proceedings, while imposing on interpreters the pressures of capturing the horror accurately and fairly, before the speaker had completed the sentence.

Sig’s road to Nuremberg was not direct. In March 1938, as a 14-year-old Austrian Jewish schoolboy, he watched Wehrmacht troops enter Vienna, observing through drawn curtains the coming of the swastika and the jubilation of a multitude of joyous Austrians.

The family were soon thrown out of their home and shortly after Kristallnacht Sig travelled to London on the kindertransport, to live with his uncle near Hampstead Heath in north London, a period he recalls with much fondness. Towards the end of the war, in 1945, Sig signed up with the US air force to work as a linguist in Germany. He learned of the trial of Nazi leaders, went Awol, and hitched a ride to Nuremberg’s palace of justice.

Within days he was sitting in a small room with Hans Frank and a military interrogator. Without any training, he interpreted the pre-trial interrogations of the man who ruled large parts of occupied Poland, the “Butcher of Warsaw”.

Frank was governor general of occupied Poland, as well as Adolf Hitler’s personal lawyer, a man charged with – and then convicted for – the murder of three million Jews and Poles. Sig remembers him as an “interesting and impressive” man “overtaken by fanaticism”. He acted “with a clear mind”, he says: “He knew he had done wrong.”

How did he feel to be in the same room as Frank, given that he had lived through Kristallnacht? That wasn’t the issue, says Sig. “I was preoccupied with doing a good job, with unfamiliar vocabulary” – the search for accuracy. “We were there to interpret, not to judge. Did subconscious, negative feelings intervene? The predominant question was not of feelings but a linguistic question, how do I accept this challenge of words.”

During questions from the audience after his talk, someone asks whether the interpreters were traumatised by what they heard. “The things we saw were shocking,” Sig says, “but they could not be translated into feelings, because we were not in a position to feel one way or another. I was 22, I just concentrated on the job”.

After the pre-trial interrogations came the main trial. Sig was there from day one – 20 November 1945 – to the end, when the sentences were handed down. Ten times he heard the presiding judge, Sir Geoffrey Lawrence of the English court of appeal, speak the words “Death by hanging” – Tode durch den strang: a straightforward matter for the interpreters. That last session was not filmed, to preserve the dignity of the defendants. He recalls many of the big moments over that year: Robert Jackson’s “unforgettable” opening speech (four nations who chose to “stay the hand of vengeance and voluntarily submit their captive enemies to the judgment of the law”); Jackson’s near disastrous cross-examination of Hermann Goering, repaired by a “brilliant” David Maxwell-Fyfe; the Bergen-Belsen film of “gas chambers and unspeakable cruelties”; the acceptance of a little responsibility by Hans Frank and Albert Speer (“they pronounced a collective guilt of Germany as a whole but would not accept individual guilt”).He recalls too the moments of levity: the festivities at the Grand Hotel, the excessive drinking by Russian officers, the buffoonery of Hermann Goering. The questions take him back to his own emotions. Yes, he says, there were moments when some of us got into difficulty. One of his colleagues was Virginia von Schon, a librarian and most talented interpreter, also “beautiful, prim and proper”. “She was on an English microphone”, Sig tells an audience on tenterhooks, “when a word came up that she could not bring herself to pronounce, because it was so vulgar”. Not wanting to pronounce it in open courtroom “she interpreted all the way up to the word, then she stopped, she just wouldn’t do it.” Sig pauses. “I took the microphone and used that word, in fact I made it worse.” He pauses again. “On that note, we might adjourn!”

The legacy and lessons of that momentous year were “extremely important”, personally and globally, Sig says. Over the decades a question recurs. “How it is possible that these things happened in a country that produced musicians, a Goethe, a Schiller, how was it possible that a culture like this could sink into the abyss into which they had fallen under the Nazis?” Sig still asks himself that question. The answer? “I attempt a response, that when you live in a society with no checks on behaviour, no acceptance of any rule of law, no respect for rules of procedure, then those things can happen in any country.” He pauses, looks up, around the room. “It’s not only a German problem, it’s a human problem.”

He feels strongly about global cooperation – he still works at the East-West Centre in Hawaii (an organisation founded in 1960 to strengthen relations between nations) – and human rights and international criminal justice. The Nuremberg judgments engendered the Universal Declaration on Human Rights, and then two years later, the European Convention. These, he says, are matters of pride and also concern: “Just because it doesn’t all work perfectly doesn’t mean it was the wrong way.” In his view Nuremberg “created a path to the rule of international law, a means for dealing with guilt and the acceptance of responsibility”, a way of distinguishing right from wrong.

It seems that some Conservatives – the justice secretary among them – wish to be rid of the European court. Chris Grayling would do well to spend an evening with Siegfried Ramler, who could refresh him on the rationale for the European Convention and its court, a system of collective security. Sig knows a thing or two about history, and what happens when checks and balances are cast aside.

• Philippe Sands’ A Song of Good and Evil, about the Nuremberg trial, will be performed with Vanessa Redgrave, Laurent Naouri and Guillaume de Chassy on 29 and 30 November, at the Purcell Room, South Bank, London.

Nuremberg and Beyond: The Memoirs of Siegfried Ramler, is published by Ahuna Press.