The Royal Spanish Academy itself (RAE), in its Panhispanic Dictionary of Doubts, says that it is still valid to write "Mexico" or "Mexican." But the recommendation for this place name (place name) is to use the letter "x".
"In Spain, the usual graphics until recently were Mexico, Mexico, etc. Although the forms with j are also correct, the spellings with x are recommended because they are used in the country and, mainly, in the rest of Latin America ", explains the RAE.
But why is Mexico written with X and not with Jota, as would be more natural given the sound of the word? "What should be known is that it is a matter of almost national identity for the origins of this.
But it is mainly due to how Spanish was spoken at the beginning of the 16th century, when the conquest of the Aztec nation (or Mexica, to be more precise) occurred.
"Méshico"
The place-name "Mexico" has its origin in the Nahuatl language spoken by the founding Aztecs of the pre-Hispanic nation. Historians point to the founding of the empire in March 1325.
The meaning of the word "Mexico" means "place in the navel of the Moon". The word "metzi" means "moon", "xictli" is "navel or center", and "co" is the Nahuatl suffix for "place". The Aztecs founded what is now Mexico on Lake Texcoco, called "lake of the moon."
Pronounced in our current Spanish would sound like "Méshico", that is, the sound of the X would be / sh /, as when the anglicism is said to show. "In ancient Spanish or medieval Castilian there was what is known as the dull prepalatal fricative sound, which is the / sh / sound".
King Alfonso X of Castile, called "El Sabio", established the Alfonsí Norma of writing in the 12th century, which established that the / sh / sound should be written with x. When the Spanish conquistadors arrived at the beginning of the 16th century, they tried to adapt the sounds of Nahuatl to Spanish at that time and used Norma Alfonsí for "Méshico", that is, Mexico (at that time without a tilde).
When does it become "Mexico"?
As early as the sixteenth century of the conquest of Mexico (1521), Castilian was losing the fricative prepalatal deaf / sh / and in the seventeenth century ended up disappearing. There are few words in our language that still have that sound, many of which come from other languages that do, such as English, Portuguese or French.
At the same time there was the flaring velar sound deaf, which is what is heard on the jota or with the ge. "In the seventeenth century nobody spoke the / sh / in Spanish, that's why in that century, everybody said Mexico (with the sound of j)".
The RAE established in 1815 the Spelling of the Castilian Language in which instructed that all the words that were written with X and that were pronounced with / j / should now be written with jota, and no longer with X.
That's when Mexico becomes Mexico for the RAE.