I don't know about the history of English. But for Persian, it's not that the language has not changed since 10th century, it is that the last revolution occurred between 7th and 10th century and all changes since then have been evolutionary. A modern reader equipped with a dictionary can read any text written in 10th century and later, but anything before that requires essentially learning a new language.

The reason for the revolutionary change in Persian language was the Muslim conquest of Persia in 633 CE and the introduction of Arabic language and culture to Iran. Anything before the conquest was in a language unlike the modern Persian. For two centuries following the conquest, there is little written Persian available, so we don't know how exactly the transformation occurred. What emerged after the two centuries of silence was a language based on Old Persian but with many Arabic loan words and written in a modified Arabic alphabet. This language is comprehensible by modern readers.

It should be noted that comprehensible does not mean easy. Reading old poems for modern Persian speakers is as difficult as reading Shakespeare is for modern anglophones, but it is a far cry from reading Beowulf.

The continuing popularity in Iran and the Persophone world of Classical poets such as Hafez and Rumi, and of poetry in general, has no equal that I know of in the West. I suspect that this tends to slow down language change to some extent. People continue to know the old texts because they are constantly exposed to them in vital, modern contexts. Perhaps it's something like the position that Shakespeare and the King James Bible held in Protestant English-speaking lands, though they were widely understood for a shorter length of time (maybe 350 years?) than the Persian classics (700-1000 years and counting, as far as I can tell). I'm afraid that Shakespeare and the Bible are now slipping beneath the waves of comprehension for non-specialists.

It may be worth adding that the adoption of the Arabic alphabet was a major part of the transition from Middle Persian to New Persian, beyond language change itself. People are often surprised by how little difficulty they have parsing a sentence in Middle Persian once it’s been transliterated.

Language development changes in speed with the linguistic landscape. Persian saw its fastest changing in the 8th century during the Ummayad conquest, when Arabic became the dominant language of the ruling class.

Similarly, English saw its fastest changing in the 12th century during the Norman conquest, when French became the dominant language of the ruling class.

In both cases, most of the core vocabulary and grammar survived, but with enough influence and new words that it is hard for speakers after the change to understand writings before the change.

My understanding from Persian friends is that Rumi is about as understandable to modern Persian speakers as his contemporary, Chaucer, is to modern English speakers.

Whan that Aprille with his shours soothe

The droughte of March hath perced to the roote

And bathed evry vein in switch liquour

Of which vertu engendyred is the flour...

Yes, but on the other hand, original lyrics by Hafez, Rumi, and others are commonly used, unchanged, as lyrics for contemporary Persian songs, even for full-on rock songs, dance music, love songs, etc. I haven't heard any songs in the Hot 100 using lyrics by Chaucer lately.

In part, this is probably because old Persian poetry/song traditions were more modern, compared to english poetry from the same period. They had a strong, very old literary tradition. The English didn't, until a bit later. Chaucer was part of building it.

The musical/poetic sensibilities of old/middle English are way too different to ours.

The Bible (especially in Hebrew) can also easily be plagiarized, and you'll find lots of lines/poems that work "as is" in a pop song.

Eg, rivers of babylon, boney m.

As a cultural reference, poetry is such a big deal that the largest and most popular tv station in Kabul (Tolo TV), operating in the Dari dialect of Farsi, regularly hosts what could basically be described as poetry slams during prime time tv hours.

They have a fairly active youtube channel if anyone wants to take a look.

The difficulty of Chaucer for modern English speakers is exaggerated because of the unfamiliar spelling. The reason that modern editions don't modernise the spelling is that if you do this it becomes hard to figure out the rhythm. For example, in the Ellesmere manuscript [1], line 9 is "And smale foweles maken melodye", but if you modernise this to "And small fowls make melody" then you only have seven syllables and it is hard to figure out how this was ever scanned as iambic pentameter. Modern editions end up compromising between spelling and scansion, making the language more difficult-looking than it really is.

[1] https://www.bl.uk/collection-items/ellesmere-manuscript

Not exactly correct. I have the 'Divan'e Shams'e Tabriz' open in front of me and even though I left Iran at 14 years of age I can read it.

The content matter is not super accessible but that was true from the very day it was uttered.

Ferdowsi's Shahnameh epic played a vital role in keeping the the Persian language alive throughout the Arabic conquest.

Languages change at vastly different rates. A new creole in its first generation of native speakers can evolve to almost unrecognisable by the second. Meanwhile, a language can be remain intelligible for millennia and many do.

Literacy, sacred texts and political stability are factors in language stability. So is the "one damned thing after another" path of history.

But to your question... I'd say English is the "outlier" or at least the example that needs explaining.

Old English isn't really English. A better name would be Saxon. It's far closer to other languages from the north sea mainland, and not at all similar to modern English. Frisian is still spoken, for example. It's basically a sister dialect to old English. Saxon was spoken by the "Anglo Saxons" who migrated to/colonized to Britain in the early mediaeval period. They barely wrote (Alfred's chronicles being a notable exception), and literacy was mostly Latin anyway. They were also a minority language for a long time.

Then the "English" (Anglo Saxons) lost political power to the normans. French came with the Norman Empire and Latin was the written & liturgical language. Various dialects of "Middle English" became "regional" languages alongside Irish/Scottish, Welsh/Cornish, some Danish, and possibly others. A lot of French got mixed in.

Eventually the Norman empire subsided or evolved into the British one, settled in England and "went native." They became the English nobility, and their posh dialect became early modern English (like Shakespeare).

Underlying a lot of this instability is the fact that Britain, Saxony and NW Europe generally was barbarian country. They (we) didn't live in cities, were borderline illiterate. Etc.

Old native traditions (ie, not Christianity) were remembered in epic songs and sagas, not books.

Languages evolve and get pushed up and down the pecking order much faster in barbarian country. Barbarian culture usually evolves much faster and more flexibly than conservative civic culture.

The parts of Europe colonized by Rome (the British outpost doesn't count) speak Latin today. None of the (more numerous) Roman conquests in "the east" speak Latin. The kept speaking Greek, Egyptian, Aramaic and such... languages that had been spoken and written in capital cities for thousands of years... even back then.

You forgot Old Norse/Danish. "Something is rotten in the state of Denmark!"

Couple that with that Saxon and Norse are Germanic. Modern English is an interesting mix in so many ways, I believe it is so pretty because it got all the rough edges polished off by this incredible meld of what made it up.

It's similar for Icelandic, 1000 year old books can be read if you know the modern language. It probably just has to do with which books are read in elementary school. I imagine that in Iran they read Rumi and Hafez a lot in elementary and high school.

The ease that modern Icelandic speakers have in understanding old texts is not due to Icelandic remaining unchanged all that time. Rather, Icelandic began to experience drastic Danish influence at a certain point, and then Iceland’s cultural elites chose to fight this by standardizing Icelandic on the basis of those old texts. It was a return to sources, not preserving those sources the whole time.

I think Persian and a handful of other languages are the outliers. The main difference between old, middle, and modern Persian is the writing system.

Old Persian is quite hard to read and understand for a modern Persian speaker, but if one spends a few weeks learning the writing system of middle Persian (called Pahlavi), they will be able to understand a lot of texts written before 500 AD.

https://en.wikipedia.org/wiki/Pahlavi_scripts

Same with Armenian: I can read and understand 5th century texts simply as an educated native speaker. It probably has to do with how much the culture relies on its written texts (because there are spoken dialects of Armenian that I dom't understand at all, while having no trouble with the written dialect from 1500 years ago)

This also applies to Arabic. It has had a long lasting preservation movement that is motivated by religion. The Quran in its original form is mostly understandable with only some additional vocabulary. Most of Quran study for Arabic speakers is about context and interpretation.