Origin of the word "El" in Spanish

No forgot the mozarabic (one ancestor of spanish)

extract:
mozarabic:
Mio sîdî ïbrâhîm
yâ tú uemme dolge
fente mib
de nohte
in non si non keris
irey-me tib
gari-me a ob
legar-te

spanish:
Mi señor Ibrahim,
¡oh tú, hombre dulce!
vente a mí
de noche.
Si no, si no quieres,
iréme a ti,
dime a dónde
encontrarte.

english:
My lord Abraham
oh you sweet man
Come to me
at night.
If not, if you don't want to,
I will go to you,
tell me where
to meet you.

this language was great influenced by arabic, most than spanish, because mozarabic was a minority language (and culture) in al-andalus, the original Latin was gradually transformed, undergoing the Arabic influence, so in this text you can see "sîdî" which comes from western Arabic (sayyid in other dialects) "sir", the Spanish "mi" become "mib" in mozarabe, is certainly an influence of Arabic.
The spanish is a descendant of this language (not directly)

"This is not found in French or Italian or Romanian. It's as if the Spanish reversed the letters in the word "le" and produced "el". How did this happen?"

As a personal pronoun, 3rd person singular, "El" exists in Romanian too and it has the same meaning as in Spanish. But it does not exist as a definite article (e.g. el Presidente) if that's what you make reference to.

If you take a look at a map, you will see the arabic articles.
Some cities towns have the article "El" and yet others have the article "Al". In two maps, the same town is shown with different articles "El-Minya" and "Al-Minya"

If people in moorish Spain were hearing both, "il" and "al", "el" seems to fit naturally.