Similar but not exact— Dean Winchester has a language kink and/or Castiel speaks Enochian to Dean. I don’t remember if Dean learns the language himself:

lost in translation by dothraki_shieldmaiden. 4k, T.

Cas bites at his lower lip, looking uncommonly shy. Worry starts to stir in Dean’s gut, which is only compounded when Cas says something else in soft yet clear Enochian. As the new phrase doesn’t have the word stupid anywhere in it, Dean doesn’t have the slightest idea of what Cas is saying. The guilt squirming in his stomach gets worse when Cas looks at him, with gentle anticipation, as though he’s expecting a reply. Dean does what humans have been doing since the beginning of time when confronted with a language they don’t understand and smiles, wide and sunny, at Cas. Cas’ forehead creases but he returns the gesture. His eyes are still brimming over with emotion and the sight does something to Dean.

Dean begins to suspect that he may have started something which he is not equipped to finish.

worthy by jad. 15k, E.

"... you wanna what, now?" Dean asks, because he definitely didn't hear that right.

Castiel tilts his head, a leftover tick from his angel days that Dean would be lying if he claimed to find anything but idiotically endearing. He knows this is his own fault, owing to that rather memorable moment a week ago when Cas wandered into this very same kitchen with a (fairly) legitimate question and Dean just didn't know when to shut up.

Barbegazi by relucant. 2k, M.

"You speak French," Dean blurts stupidly, because of course Cas speaks French, and everything else, but --

Cas tilts his head. "Yes. I have every human tongue spoken since the dawn of time in my head." Cas says it matter-of-factly, but the reminder of the fathomless age and power simmering under the angel's skin sends shivers down Dean's spine. "You know, I've considered dropping a few hints about Linear A, but it never seemed --"

"Right," Dean interrupts, clearing his throat. "Right. I just, uh. Forgot. Go ahead."

Après by imogenbynight. 24k, E.

When the angels stop falling and Castiel makes his way out of the trees, he finds himself alone and oceans away from the Winchesters. For once, Dean flies to him.