Language is a prominent motif in The Tempest. In Act 1, Scene 2, Caliban curses Prospero for teaching him his language:
You taught me language; and my profit on't
Is, I know how to curse. The red plague rid you
For learning me your language!
He curses the man who taught him to speak because language allows him to understand his otherness. He feels trapped by the requirement to use the language of the captor who has deprived him of his freedom. He feels frustrated that this new power seems significant to Prospero but still leaves Caliban himself powerless in the ways that really matter.
Language also signifies the idea of civility—Caliban despises that he remains subordinate to Prospero despite having comparable linguistic capacity. Language is the single privilege Caliban has, and as he aptly observes, it does him very little good beyond allowing him to curse the rest of his circumstances. Shakespeare often makes the characters with high political status speak in beautiful verse, and the lower-status characters speak in simple prose, but Caliban shows great range in his capacity to put words together.
This is, then, an excellent example of situational irony because the very thing that many people assume would "civilize" Caliban ends up making him even more bitter and prone to cursing (which is the opposite of what most people would consider civilized behavior).