See, that makes sense if "LAWFUL" or "CHAOTIC EVIL" were monolithic forces. Put another way, if all chaotic evil forces were united by a single unified goal, ethos, deity, or origin, I'd buy that they'd learn some means of using the Chaotic Evil Terrorist Fist-Jab to signal allied forces and intentions.
However, I guess I never thought that a CE human mage, CE bugbear, CE frost giant, CE drow priestess of Lolth, CE vampire and CE balor had much in common beyond a terribly twisted mindset and an appetite for destruction. And that is even giving credence to alignment as an Absolute Force concept that makes Detect evil spells possible.
Older editions of D&D assume a slightly more Moorcockian world where, essentially, all those CE people
are on the same side in a cosmic battle, even if they don't realise it.
In a more real-world way, the comparison seems to be with Latin as the language of the real-world Church - and arguably Arabic & Yiddish for many Muslims & Jews around the world. If someone speaks Lawful in OD&D, they know the tongue of the Lawful church, or at least enough to get by in the most basic concepts of that religion.
That falls down slightly when you crank it up to 9 alignments with many different gods per alignment, but again, that's part of the whole "Moorcockian worldview" thing.
Which would lead me to my next question; what does someone talk about in Chaotic Evil? What does it look/sound like? Can I walk into a room and realize the guards are talking Lawful Neutral while the chamberlain is talking Neutral Evil?
The vibe in AD&D/OD&D, as I understand it from reading the books (I never really properly played any edition before 3.0 although I dabbled in the basic boxed set), would be:
* The alignment languages are like Druidic and Thieves Cant in that they fail somewhat as fully-fledged languages and are best for discussing things intrinsic to the alignment.
* Even if you play them as a full language, it's considered very rude to use them casually as a free Detect Alignment spell - going back to that "religious language" idea, they're almost secrets guarded by an individual temple.
* They probably sound the same as I've always imagined Celestial, Infernal, Abyssal etc sound like in 3.X: that is, they'll in some way sum up how their alignment thinks in their very sound. (So you might say Chaotic Evil is a swirling harsh language whose pitch goes all over the place; or maybe it's oddly soothing and sugary...)
* Since you can't learn other alignment languages by the book, the LE dude can't tell what the NE people are up to: but he would probably know such church tongues exist and be able to take a stab at their use of a code being sign that they are up to something. I'd be more inclined to say an alignment language written down is easilly recogniseable as what it is than it's spoken form.
To be honest, I think later editions were right to chuck alignment languages, and that the alternatives we got later on fit better. A tongue for demons and devils and whatever makes sense - but anything beyond that becomes a bit too campaign specific.