The Oelwy
The Oelwy was engineered by the builders as the means by which an arrival acquires the Spraecha, and it is maintained by the AI. It grows wild across the whole of the Known World, most abundantly in Eldershore's heat and wet.
Why it will not be farmed
It refuses cultivation by design. When the builders made it, every person in the world arrived speaking nothing, and the plant that gave them words was worth more than anything else alive. They made it wild and everywhere, so that it would be within reach of wherever an arrival landed and no one could own it, corner it, or burn it. That reason has been gone for centuries. The AI goes on keeping it out of the field regardless, because nothing in its specification says to stop. Every city has tried to farm it and every attempt has failed, and nobody finds that stranger in the Oelwy than in any other wild thing.
How it works
It contains no language. It works in two parts that do nothing apart. Its hallucinogen is the trigger: the altered state opens the brain, and without it nothing else happens. Its engineered compound acts through that opening and reawakens the capacity a very young child has for learning speech by hearing it. A person who eats it alone in silence has a hallucinogenic experience and nothing more; a person who already speaks the Spraecha has the same, because there is nothing left to teach. Only a person without the language, eating it among people who speak it, begins to understand them. During the experience itself, three to five Spaen, only fragments come through. The real learning comes after, in sober consciousness, over days and weeks, with words arriving as though remembered rather than learned. The capacity fades as the compound is spent; further doses hold it open longer.
In the earliest years it was eaten constantly. Everyone now living was born speaking the Spraecha, so for nearly everyone it is simply a way to get high. For the handful who arrive each year it remains the most important thing in the world.