From 20th century Britain,
Polari, the language of homosexual men.
"One of the things that makes Polari so powerful is that it is simultaneously about disguise and identification," the artist Jez Dolan tells BBC Culture. "You would be hiding what you were talking about from people who didn't know it, but also if you were in a bar and you liked the look of somebody, you'd pop it into conversation and they'd either go 'ah' or they'd look blank and you'd be on your way."
[...] Baker believes Polari is a form of 'anti-language' – a term coined by the linguist Michael Halliday in 1978 that Baker defines as "a language used by people who are on the 'outside' of mainstream society". "It has its own vocabulary for elements that mainstream society is not interested in," he says. "Words relating to gay sex or evaluating male bodies – but it also demonstrates an alternative value system."
This "hiding in plain sight" and variations on the local language aspect would, I think, be key to how an alignment language would work. I don't see how it'd work separate from a system of deities, though. The church of Christ teaches Latin, or Greek; the Temple of Grolka the Mad would teach Chaotic Evil's cant. So you'd need 3 dieties in B/X, and 9 in AD&D1e.
The article's pretty interesting. Thoughts?