I'm just overthinking things here, so feel free to junk it as too much baggage from my linguistics classes, but my 2 bits on the language discussion:
I think acknowledging dialects is fine, but in terms of actually listing them as specific languages for a character, I just wind up subdividing too much to make them useful. For example, part of your dialect would be regional (Northern, Southern, Boston...) part socio-economic (upper or lower class grammars are usually noticeably different, not least of all due to schooling), and part occupational (engineers and psychiatrists aren't exactly equipped to effectively discuss each other's disciplines, and your career inevitably influences your worldview and thus your word / metaphor choices). Then there's the fact that most people shift dialects depending on the company they keep (As a minor example, most people swear noticeably more or less with their parents or when they're in professional settings).
I think in terms of basic dialects, all that's easier to do just through dialogue than a game mechanic. I like the idea of some of the specialized languages, though. A thieves cant would be something like Druidic: a language designed to provide a secure linguistic interaction for a secret society. Here, so thieves can talk about their jobs without the local authorities honing in on them. Gutterspeak, too, makes sense as a kind of creole / shorthand developed by a fairly isolated group (and, too, this might have its own elements of "we don't want others listening in").
And if we want to say that Metropolis is large enough (or has enough sections that effectively declared themselves isolated entities in the distant past) to have significant linguistic drift, a trader's creole also makes sense. I'd think we wouldn't likely need it in game play, though, unless we wound up crossing all the way from one end of the never ending city to the other.
That was way too much rambling. The short version: I'd say dialects as you want to roleplay them, but not really listed on the sheet. Specialized languages (theives cant, gutterspeak, possibly trade creole) as warranted by circumstances.
I could also be entirely full of it, but that's often a given with me.
jason