Pokémon is well-known for having some baffling designs and strange lore. Gen I may have been bare in bizarre lore, but from Gen II on, the background and culture of the Pokémon world has been expanded, not only through the Pokédex but also through the overworld and other aspects of the franchise. One Pokémon has a great deal of mystery surrounding it, despite appearing repeatedly from Gen II onwards: Unown.
Unown may be intrinsically linked with the development of language in the Pokémon world, but it's unclear how. Unown’s various Pokédex entries state that even after dedicated study, linguists in the Pokémon world aren't sure if Unown developed before or after the advent of written language (and Pokémon do just spring up out of nowhere -- like the Klink line).
At the same time, some Asian mythology credits dragons with teaching people how to read and write, and the Sinnoh region itself delves deeply into this mindset, with Pokémon like Uxie, Mespirit and Azelf being credited with teaching humans about intelligence and emotion. Considering how connected Johto and Sinnoh are in the games, it makes sense that these Pokémon would be at least marginally credited with the origin of written language.
That said, Pokémon and human language don't seem linked and an old, circular theory regarding Pokémon names suggest they were named after the noises they make (more sensible with the anime, less so with the games). If the written language is based on a singular Pokémon, this would explain the homogeneity of language within the Pokémon world. However, Unown have been shown in the shape of Greek and Cyrillic symbols, suggesting a variety of languages and, by extension, a variety of Unown.
Granted, this issue stems from a divide in continuity between the anime and the games. Unown in-game do not have specific noises for their specific shapes. In the anime, they simply say, "Unown."
How then could the Unown teach that their shapes have specific meaning? Their Psychic typing would suggest telepathy, which is what their in-game Pokédex entries state they’re capable of. Yet, Professor Oak's notes at the beginning of the third movie suggest Unown are incapable of using telepathy to communicate, despite being capable of altering the world based on brain waves.
This ability, perhaps, suggests a means to teaching a written language to a sufficiently advanced people, especially when coupled with the knowledge that Unown are almost exclusively found in ancient ruins. These could be environments created by ancient Pokémon as thanks to their mysterious, alphabetic benefactors.
Considering Sinnoh’s lore, these ancient civilizations may have met Uxie and Unown at similar times -- perhaps the two Pokémon species together enabled the understanding of Unown's shape as a cohesive language. Unown and Arceus are required to work together to access the Sinjoh ruins, which offers support for two Pokémon species working together to benefit humanity, as does the literature found in the Canalave Library in Sinnoh.
Perhaps there's another explanation. Unown are said to come from another dimension and prefer to live there. In Omega Ruby and Alpha Sapphire, they can only be encountered through Mirage Spots -- or rather, dimensional rifts. Unown possess powers that warp the universe and have abilities beyond understanding.
These attributes bring to mind another group of classified Pokémon that don't appear until Gen VII: Ultra Beasts. Ultra Beasts, such as the Cosmog line, can have some beneficial qualities, so it stands to reason that another mind-boggling Pokémon can have positive qualities to deliver to humanity as well.
Perhaps Unown is the most accurately-named Pokémon in existence. They're tied with written language, though it's unclear how; if they taught humans written language, there's still that overarching question. This does reflect questions surrounding how verbal and written language are entangled in the real world, which makes sense, even if Pokémon don't also exist in reality.