Nota Bene: Aklo is actually a fictitious language from another, pre-Pathfinder mythos - or should I say Mythos, because it's the name given by some to the language of Lovecraftian creepiness ("ph'nglui mglw'nafh cthulhu r'lyeh wgah'nagl fhtagn" being the sample sentence we all learn in Aklo 101). Thus, for Aklo and Elven (and possibly Orcish, Dwarven, and maybe Halfling, since Tolkien did exactly this kind of thing), you've already got ready-made, unique languages in their own right, well-constructed enough to practically be considered real languages (there's also Klingon, of course, and evidently, Na'vi as well now - I agree Klingon would be good for Orcish, and Na'vi might be good for Shoanti or Sylvan).
Note on Celestial: 100 years ago, a thing called "Enochian," supposedly the language of angels, was all the rage. It was honored with a passing reference in City of Villains, and here's a clip of Aleister Crowley giving us a sample!
If I wanted to construct an Infernal language, I'd take Enochian and violate it in the Orwellian Newspeak fashion, shrinking, simplifying, and mutilating it until it was incapable of conveying non-Lawful Evil concepts.
Yeah, I pretty much use all of those languages as counterparts in my games. Celestial is Enochian and Infernal is a corrupted version of it (they use the same alphabet, after all).
Another made up language that from what I hear is fully realized, although only two people fully speak it, that being Luc Besson and Milla Jovovich. The language Leeloo speaks in The Fifth Element. It's said they would have full conversations on set and the other actors were like "O_o"