So, I don’t know anything about Old Spanish (sorry) or even if it’s called “Old Spanish” officially, but I do study Old English, and the short answer is you can’t read it because it’s basically a different language than Modern English.

Old English was spoken/written from about 800-1100 in Britain. It is similar to Old Norse and Old High German and also borrows some grammar from Latin. For a good example of Old English look up the text of Beowulf. Old English is the progenitor of English though, which is why some stuff is familiar. “Wife” for example, was “wīf” in OE (pronounced “weef”)

So then the Normans invaded in 1066 and that’s when Old English started to morph into Middle English, which was used from about 1100-1400. BTW, all these dates are rough approximations. Language transitions are long processes that happen at different times in different places. Anyway, now the word “wife” is “wyf” but pronounced the same way as before. During Middle English the Great Vowel Shift happens, so now words are pronounced differently, which leads to them being spelt differently. Middle English has LOTS of variations is spelling (to the point where you can tell where something was written, because the specific regional accent is apparent in the spelling), more so than even Old English, because of the GVS. For a great example of Middle English look at Geoffrey Chaucer‘s Canterbury Tales. A highly literate English speaker can read Middle English without training, but not very well.

Anyway, eventually language moves toward standardization and by Shakespeare it is officially “Early Modern English.” This is when English really becomes readable to the average modern English speaker. Still difficult, but it’s no longer a different language entirely.

Edit 5- it was pointed out in the comments that Shakespeare is not fully standardized, which is 100% correct, and I forgot to mention it. I’m pretty sure there are even variations between the folios. So when you go to the store and buy a copy of Shakespeare (or read the text I copy/pasted below) I should have noted that the spelling is cleaned up by an editor. It’s still Early Modern English, but it’s not fully standard.

Hope that helps!

Edit- the Normans were speaking French and that’s why it kick started a major shift in language

Final edit! Also don’t think of English as being modern now, so it’s done changing. It’s still changing- it hasn’t stopped. New words appear all the time (yeet, fleek, pwned), or old words are used in new ways. (tablet, text as a verb). Also emojis have also been fully integrated as part of written language 🤷🏼‍♂️ so someday philologists may say that a new phase of English (post modern, maybe?) began around the year 2000 with the dawn of the Internet age. Don’t think of language or history as being “finished” is what I’m saying

Edit 2- if you’re curious, here is the first bit of text from Beowulf:

Hwæt. We Gardena in geardagum, þeodcyninga, þrym gefrunon, hu ða æþelingas ellen fremedon. Oft Scyld Scefing sceaþena þreatum, monegum mægþum, meodosetla ofteah, egsode eorlas. Syððan ærest wearð feasceaft funden, he þæs frofre gebad, weox under wolcnum, weorðmyndum þah, oðþæt him æghwylc þara ymbsittendra ofer hronrade hyran scolde, gomban gyldan. þæt wæs god cyning. ðæm eafera wæs æfter cenned, geong in geardum, þone god sende folce to frofre; fyrenðearfe ongeat þe hie ær drugon aldorlease lange hwile. Him þæs liffrea, wuldres wealdend, woroldare forgeaf; Beowulf wæs breme blæd wide sprang, Scyldes eafera Scedelandum in.

Edit 3- and this is my favorite bit from the General Prologue of Canterbury Tales:

A CLERK ther was of Oxenford also, That unto logik hadde longe y-go. As lene was his hors as is a rake, And he nas nat right fat, I undertake; But loked holwe, and ther-to soberly. Ful thredbar was his overest courtepy; For he had geten him yet no benefyce, Ne was so worldly for to have offyce. For him was lever have at his beddes heed Twenty bokes, clad in blak or reed, Of Aristotle and his philosophye, Than robes riche, or fithele, or gay sautrye. But al be that he was a philosophre, Yet hadde he but litel gold in cofre; But al that he mighte of his freendes hente, On bokes and on lerninge he it spente, And bisily gan for the soules preye Of hem that yaf him wher-with to scoleye. Of studie took he most cure and most hede. Noght o word spak he more than was nede, And that was seyd in forme and reverence, And short and quik, and ful of hy sentence. Souninge in moral vertu was his speche, And gladly wolde he lerne, and gladly teche.

Edit 4- and just for good measure here’s some Shakespeare:

Chorus. Two households, both alike in dignity, In fair Verona, where we lay our scene, From ancient grudge break to new mutiny, Where civil blood makes civil hands unclean. From forth the fatal loins of these two foes A pair of star-cross'd lovers take their life; Whose misadventured piteous overthrows Do with their death bury their parents' strife. The fearful passage of their death-mark'd love, And the continuance of their parents' rage, Which, but their children's end, nought could remove, Is now the two hours' traffic of our stage; The which if you with patient ears attend, What here shall miss, our toil shall strive to mend.

/u/RickFletching Pretty much said it all, but I do want to point out that this Spanish poem is a little more contemporary with Early Middle English than Old English. It might be better to compare something with The Owl and The Nightingale, an Early Middle English poem. Note that El Cantar Del Mio Cid is (from what I can tell) from Old Castillian specifically, while this poem might come from a dialect that didn't evolve into Modern English.

Anyway, you can see that this is still quite hard to read, though it's easier to tease out a meaning than Beowulf. It's also not nearly as easy as Chaucer, which is difficult, but more what I think of when you describe reading El Cantar Del Mio Cid.