I have in search for a fic for WEEKS and to no luck. In said fic, Tony is a sort of octopus creature that had been kidnapped and experimented on by Hydra. The team finds Tony and brings him with them as there's some law about sentient beings. Once at SHIELD, Tony is found to be very intelligent and understands English. Steve and Tony become close friends as they are both in a foreign world, and I think the fic ends with Tony giving his report or something, but don't quote me on that one.
*SOME RANDOM TIDBITS THAT MIGHT HELP*
-At one point Tony shows Steve how his language works, something like octopus sign language.
-There's a moment where Tony is talking about drinkable water, and Steve says something along the lines of "You should have told us! It's, well, illegal."
-Tony does a lot of communicating by writing on his glass cage's walls with marker
You'd think that with all this, I would have found it, but alas.
Thank you, and good luck!