This time, I’m asking if there’s a point in history, where (almost) all non-English speaking people could get killed off by a natural disaster?
In response to the question as asked:
not if we're talking "real world" history and related real world possible "natural" disasters rather than a fantasy history/speculative future science fiction "history" where the circumstances can be highly contrived or things can exist like diseases which can be transmitted and correspondingly combatted cognitively/semantically/semiotically rather than physically/biotically/biologically. I, at least, can't honestly think of even a remote chance for "natural causes" in real world history that somehow leaves such a predominant English speaking population as could be described by "(almost) all non-English speaking people could get killed off...".
Even the basic idea that it might be remotely reasonable enough to be believable honestly easily comes across as... concernedly... anglo-centric/euro-centric/american-centric of some kind. The world and world history are much, much larger than the impressions given by American or English history as usually taught in schools. I'm assuming that wasn't an intent in asking, I don't want to sound harsh in turn bringing it up, but I want to be open in providing the perspective that it might not sound great to some people and could bring to mind concerns over what its impetus might be, in case that's not been considered.
To be explicit expounding on "no" as an answer, anything that turns on a biological disease where "isolated communities" survive is going to have just as many or more "isolated communities" speaking other languages as English. Etc.
Extinction level events are going to run into similar issues. Anything natural that kills off such a significant amount of the world's population as to have killed (almost) all non-English speaking people is going to have also killed off such a significant number of English speakers as to result in a surviving population where they aren't proportionally as significant as the posed question implies.
There are no realistic/"natural" circumstances that change this, there are plenty of islands where languages other than English are spoken, and for anything contagious it's simply beyond unrealistic to contrive a scenario where Britain doesn't become infected if it has an R0 high enough to be quickly propagating world-wide. Even island borders are like sieves. Asteroid or similar impacts "on the opposite side of the globe" are still going to leave significant non-English speaking populations, if they leave any English speaking populations alive. Events affecting based on elevation run into the same issues. Hemisphere based events too.
I can think of no point in actual real world history where this would somehow be different in terms of "natural" disasters: if anything, it would be less likely to be even remotely possible to occur historically rather than more. Any balances of less/slower travel would be quickly cancelled by less communication and less related control/monitoring capacity of travel, and less scientific understanding/capability.
Pretzeling to find a "semi-realistic" excuse for basically everyone in a book speaking English (with that as the primary and driving cause alone, rather than a side effect of a progression of events already in motion) seems like... well, either a doomed exercise from the start that will fall short of suspension of disbelief or likely to turn into something that's going to sound very questionable to at least some readers.
"Why do they all speak English" in a novel is usually best:
- not answered in any way; it's the narration afterall, and doesn't need to be explained at all: sometimes less or even none is more.
- implicitly answered by describing language barriers between characters but providing all dialogue in English (the narrator is "translating") (particularly recommended when the narrator doesn't speak the languages in question and they aren't fantasy languages, as it's FAR better than botching the languages in question).
- not done to such an extent, by instead providing starting snippets in other languages accompanied by translation, or some similar balance/mixing. (Ideally only performed by at minimum entirely fluent and culturally immersed authors, or by contracting linguistic copy/translation/editorial help for the work to be done by someone for whom both the culture and language are native if the author is not). This is a lot of work but can be deeply enriching both to the work and the author, in multiple senses.
With that all said, a frame shift Answer that considers a human, rather than "natural" disaster:
(almost) all non-English speaking people get [horribly] killed off by:
A slightly altered historic timeline where the UK was taken over by a populist fascist leader following the first World War, while having advanced a bit over half a century's worth (of real history) ahead of the rest of the world technologically over the course of the preceding century.
Having pooled all of its resources into scientific advancement at the height of its imperial grasp and stability, and having made certain key advancements in the late 1800s, the British Empire achieved jet flight, jet missiles, transistors, transistor based computing, digital wire and radio communication, and nuclear fission power and weaponry--mostly in secret--before 1914. Some of these were used in World War I, but the leadership at the time made decisions to keep the rest secret for holding advantages and for ethical concerns (in the case of nuclear weaponry). During World War I, the concept of using extraterrestrial platforms to relay communications signals was advanced and attempts at space flight begun, under various cover stories which unfortunately backfired in terms of popular sentiment during and immediately after the War.
[making all of this work as advancements just within one country, deeply in secret so not represented in advancements in what's being taught in universities/etc to found further development will be difficult, if it is to be directly addressed. There are multiple real world parallels to far less extent in terms of technological disparity, but most of those occurred during deeply adversarial times, so leeway during the height of the British hegemon might work, especially if represented by similar security practices and drive for rapid advancement as what occurred on those wartime projects]
Held tightly in the grip of autocratic fascism following and in response to the Great War and leadership decisions during it, Britain engaged in a first strike to re-take the American colonies, obliterating the US government and major military command centers with a set of nuclear strikes, seizing resources, and putting to rest the concept of American exceptionalism being anything more than merely a laughably echoed expression of the truth in British dominance. Plundered assets were churned back into establishing a satellite communications network at the highest priority, with the understanding of how key immediate central communication would be given the Britain's previous experience with a global Empire. Initial unrest and a nascent second revolutionary war in America were brutally put to rest with following nuclear strikes on selected population centers. A clear message that there are no United States, only North American Britannia (NAB), which includes Canada.
The Empire then went on a rapidly expanding colonial genocidal spree, conquering and then killing all individuals and populations that didn't assimilate to only speak English under British rule. Because clearly "splendid isolation" had failed and the only answer to establish a lasting Pax Britannia was to make sure the hegemony was such that only that which was Britannia survived across the world. And a primary measure of full indoctrinated assimilation was mandating the use of English only, with harsh measures for any and all deviations (where does this put other UK isles languages such as Scots and Welsh and Cornish and Gaeilge and Gàidhlig and Ullans [where are we on dialect vs language?] and Shelta and Cockney [lol, I tease]? well...).
It's not a "natural" disaster, but it's a very human one with plenty of (horrific) real world history analogues to draw from and real world history to found itself in, and it folds in aspects of historical advancement that are within the realm of possibility, particularly since it relies on now-understood technological aspects rather than anything speculative scientifically.