I haven’t been able to find a literacy campaign earlier than King Sejong the Great of Korea‘s 1443-1444 Hunminjeongeum (“The Correct/Proper Sounds for the Instruction of the People”) in which he describes
“an entirely new and native script for the Korean language,” apparently of his own design.* The proclamation effectively describes the modern Korean Hangul phonetic script, which he created the express purpose of improving literacy. Here’s a translation of part of his proclamation:
Because the speech of this country is different from that of China, it [the spoken language] does not match the [Chinese] letters. Therefore, even if the ignorant want to communicate, many of them, in the end, cannot successfully express themselves. Saddened by this, I have [had] 28 letters newly made. It is my wish that all the people may easily learn these letters and that [they] be convenient for daily use.Wikipedia – Hunminjeongeum
He also comissioned a technical manual, the Hunminjeongeum Haerye, explaining in detail how the system was designed – apparently the shape of some parts of the script reflect the shape of the mouth when speaking the relevant sounds.
Great work indeed.
In an amusing little piece of irony, less than a century later Sejong’s great-grandson, Yeonsangun, went on to ban Hangul script after mocking posters about him written by commoners started appearing.**
*Let me know if you have an earlier candidate.
**They mocked him for the dastardly things he got up to, which included banning various kinds of speech and closing the royal university and a temple so that they could be turned into an enormous pleasure gardens for Yeonsangun and some of his thousand or so concubines. One can only assume he’s referred to as “the not so great”.