When I watched this cartoon, I was pretty small so I only read the subtitle on the screen ( in my mother language), so, I didn't feel annoy about that. And, I prefer the second season design to the first!
DANTE BASCO the voice of Jake Long REALLY TALKS LIKE THAT INCASE YOU GUYS DIDN'T KNOW HE'S FROM AN URBAN ENVIRONMENT. Not every one talks like they come from a suburb. Way before he did this show he was on the Fresh Prince of Bel-Air, Moesha and Fakin' Da Funk in which he played as an asian boy adopted by a Black Family, He was also Kwok, Penny's boyfriend from the Proud Family. If your a Dante Basco Zuko/Jake Long fan you should know this by now. Dante Basco is a poet/Rapper and hip hop dancer. He grew up around black people.
I believe the purpose was (having Jake talk like a "gangster") was to really show the essence of New York. Coming from a portion of the Empire State near New York City, I can personally say that some people do in fact talk in that manner. Is it annoying? Absolutely. But that's how people talk, because you learn to speak like people you're around. In simpler terms, it's a stereotype. However, there is a decently mushy-gushy reasoning behind his speech. As previously mentioned in these comments, Jake's speech evenly matches the "poseur" type of New Yorkers. The reason? Jake Long is an Asian-American dragon posing as a human. Him speaking like that is to portray his perpetual "black sheep" status. He's the American Dragon posing as a human, and an Asian trying to pose as a "gangster", though he horribly fails at it. You see, Jake Long's speech patterns are really a reflection of his status in life. It's also why he turns into his smack-talking self when he's in dragon form, and not as bad when he's in human form.
This reason is cheesy and probably not the true reason behind why Jake talks the way he does, but I always told myself that reason to keep myself from changing the channel because I do love this show.