Remember, back in the day, when every alignment had its own language? Yeah, ridiculous, and something that really doesn’t jell with logical thought. (There was also an “alignment war,” where everybody lined up and were only admitted into certain parties if their alignment was the same as the team captain’s. As always, True Neutral entities were chosen last.) There was some goofy crap back in the day, that only came out of the system later when the metagame tried to go for roleplaying immersion instead of tactical wargaming.
Making slightly more sense were the secret druid language and thieves’ cant. I liked the concept of these–by being a member of a secret society, you gain its language–but at the same time, disliked it for the same reason. The game automatically assumed you were part of the secret society, be it a druid grove or a thieves’ guild, and if you weren’t… well, you still got to speak their tongue anyway. With 3.x, and its dozens of variant rules, it’s quite easy to hedge in certain bonuses for being a part of these societies at the loss of something else (much like bardic schools drop certain skill proficiencies in exchange for others). I’d be tempted to write up a variant rogue for those who join a thieves’ guild–for example, getting thieves’ cant at first level for free, replacing most of the special abilities with things more stealthy and thiefy (too bad all the good, core thief feats are those measly +2 to two skills feats). Perhaps I shall look into this.
Still, thieves’ cant. Planescape: Torment used this to great effect; who can forget the bizaare cockney spouting from the characters all throughout Sigil? 1st Edition AD&D allowed thieves’ to speak cant, and Planescape utalized it heavily in its sourcebooks. It’s something that disappeared from the radar after a while, when people realized it made no rational sense as a requirement but didn’t have the time or memory to make it an official variant–it’s a language, for crying out loud. However, many people do still use it, or variations thereof. In Malhavoc’s Book of Roguish Luck, most of the gutter mage’s spell names utalize slang and cant, creating its own specific cant out of the rediculous slang used for spell-names. Cant itself can really give a rogue-based game depth, as long as it’s used sparingly and well.
For those still looking to utalize thieves’ cant and low-life slang, head on over to the 1736 Canting Dictionary for all your needs. It’s pretty interesting to see how lewd and low-down London was, to the point where it created its own gutter-tongue to handle the various modes of trickery, theft, purloinment, and fraud. Several of the terms have been transferred directly into modern english, which is even more interesting to realize the original meaning of the words.