“.. is to earn a new soul.”
I do not remember who first said this, maybe it was Oscar Wilde, that to Learn a new Tongue is to Learn a New Soul. But there is, increasingly, scientific evidence behind the sentiment itself – with an allowance for poetic license. To learn a second or third language is to establish entirely new pathways in the brain, in effect “rewiring” it, allowing one to see the world itself in entirely different ways. These ways may be nuanced, and hard to articulate, but they exist.
Few things are more feared by power than verbal abilities of dissidents. As for intellectual abilities, IQ, and so on well these are strongly correlated with verbal abilities. In truth having a moderate IQ and an excellent ability to convey your thoughts and ideas is far more dangerous to an elite than being a genius who is socially and verbally unable to effectively convey anything of substance to anyone outside of his recondite little circle.
The ability to effectively communicate one’s ideas, not being wordy or artistically articulate, but effectively distill meaning into verbal form and persuasively put it across, can manage to get one laid, admired, followed, and murdered. Sometimes all in the same week. History well attests to this.
Why do you think that black slaves in the American South were not allowed to be taught to read, by force of law and custom? This also existed among the ‘freedom loving and virile’ Spartans in Ancient Greece, where apart from a numerically small elite, the majority of the population were in fact slaves. (While though some craven historians refrain from pointing this out, contemporaries did note that Helots were, basically, slaves).