Going on a bit of a tangent here, but I've done a whole lot of researching and hunting and viewing of old media from the late Victorian-Edwardian era (a literal century ago) and the thing with it is is while the language and demeanor and everything has changed greatly, the humanity is the same.
They're like, the same as people today, just written about and dressed differently because social standards in general were very different. You can see it a lot in videos especially, when people are moving, and go "Hey wait, those are other people!" Because just reading about and seeing still pictures or art kinda gives the idea that people were very different or very far removed from us today when it really wasn't the case. It's really cool, actually.
One standout moment in particular was this one recording of some ironworkers making some sort of large metal disc for some sort of purpose. The company was recording them to show off a bit I think (this was still the era when video cameras were a rarity), there were a few others in the collection by the same company in different places. But I remember one ironworker in particular kept turning to stare at the camera, just sort of confused and trying to see what the hell the thing pointed at him even was (because obviously he had never seen anything like it before). I still think about that a lot. How this random guy who I'd never meet, who died long before I was born, who I'd never even know the name of, a completely unremarkable person by all historical accounts, was immortalized and far into the future, I, some kid 100 years later, could see him today on machinery he would never have been able to comprehend. What he thought was a mildly weird day and then probably not much else I can see now in fascination. And moreover, I could see him and all of his coworkers
at all, in the moment. Of course there is film degradation but I could
see it. As you said, a window in time. It's so cool.
To go back to the topic though of language (I'll just be talking about English though since that is what I know), outside of formalities and what is and isn't considered appropriate, in the grand scheme of things it hasn't changed too much since maybe the 1940s? Probably sooner but I'm going to go with a safe estimate. There are still words that have changed their meaning as time has gone on since then for example, but not enough to considerably say it an evolution in my opinion.
I feel like for it to evolve it would have to be maybe 300 more years for all of those standards and formalities to completely change. We're not far enough removed from the past to have it fully be different yet.
And as for being able to understand it, I think it would be closer to maybe like, older Shakespeare for example. A normal person can't read it very well but someone who knows the rules and words of older English can. There is way too much documentation in my opinion for it to become completely lost but to a common person it might be (or at the very least a lot harder to decipher).
I think the internet will make it change even faster, honestly. Just look at all the weird words they've made up on TikTok.
I disagree. Those are slang terms. Generally they just swap in for other words in the place (stop the cap = stop lying, fanum tax = I'm taking your food, etc). Slang doesn't change the base underlying language. If you look at say, a move from 3-4 decades ago where people use said slang terms, I feel like it is more comparable to that. The reason though why it seems like there is such an excess in comparison to previously is because 1. You're living through it right now 2. On the internet time and media moves a lot quicker than they did prior because of how fast the communication is in general and 3. Slang has always been more common amoung common people. To go back to the old media hunting, an annoying thing about researching time periods like the Victorian or Edwardian era is back then education wasn't as common so a lot of the normal people didn't know how to read and write, only the rich did. And even if someone did happen to know how to read and write, it's not like said writings would have survived. So as such the window we have is small and only from the perspective of those privileged enough to write (and have the time to write and have the power to have their records survive into the future). If you didn't know any better you'd think the past is all ballrooms and pretty ballgowns and things like that. Plus any other outside culture that is written about is biased and from the perspective of someone not living through it, which leads to lots of inaccuracies, or a lack of information on things that would have been happening that would have only been known to those living through it. This is also part of the reason why the olden times come off as so formal (I mean, people were more formal but
especially more formal), because it's primarily written about by rich people who
were more formal because they would be bankers and royals and politicians and other high up positions and the children of those people that required that formality. It's the equivilant of the only records of this time period surviving being the videos and writings of all of those fancy rich YouTubers who live in those white stainless mansions, because none of us were allowed or too broke to be on the internet. Now that we have the internet and are generally more educated, said slang is able to move around a lot more frequently
because we are the ones writing it and then using it in what is basically a virtual town, colloquially, not limited by just published media where you can't portray reality 100% unless if it's a documentary.
If we kept stacking slang on slang like now, then progressively it would change, but it still would take years and constant usage for it to become so. We don't say YOLO or Flamed/Flaming anymore but we do still say Aggro or Kiting (only ones I could think of still in use are video game related but the point still stands 😭). If the slang keeps swapping out, it will just remain slang. What turns slang into an actual word is if it is still used into the future and few of those terms will make it that long.