Summon Night: Swordcraft Story 2 features a robot who initially talks in Spock Speak... but the main character can't understand him and asks him to speak more understandably, so he starts talking in obnoxious Totally Radical speech. He later goes back to Spock Speak, to the relief of the other characters, and likely the relief of the player as well.
Jake from Advance Wars: Dual Strike. "Black Hole is all up in our business." Cue cringing. Or laughing, depending on the player's temperament.
It gets worse from there: his victory line is "Get the plates, 'cuz you just got served!"
Waylon's very first line in Days of Ruin is a deliberate self parody/Lampshade Hanging of the NoA localization team's work with the aforementioned Jake. "Would someone tell me why these Lazurians are up in my business?". He then spends the rest of the game speaking in some kind of weird 50s hipster slang.
Oracle Of Tao has a call shop item that summons really strange people (one of which talks about "fashions straight off the runway" when nobody even knows what that is) to buy and sell goods. One such guy sounds like a 1980s reject or Surfer Dude (to which Ambrosia tells him, "stop calling me man").
Pokemon Diamond And Pearl have people saying things such as "If you don't have Gym Badges, people think you're a total n00b, right?" and "I just got owned!" Translator Douglas Dinsdale of Something Awful put in a lot of Internet memes (like an artist on Route 208 saying he will name a painting "My Pokémon Is Fight") as injokes. Thankfully, much of the dialogue was rewritten in Platinum.
"Jock" type characters in Animal Crossing: Wild World also refer to you as a "total noob" if you annoy them.
Likewise, Metal Gear Acid 2 had a nerdy Playful Hacker who used Internet slang in dialogue; unfortunately, Internet slang evolves so fast his use of it seems dated just a year later.
"Dood" was an addition by the original translators by Atlus, but kept by Nippon Ichi when they opened their US branch. In the original, the Prinnies constantly end their sentences with ?? (ssu), a colloquial contraction of the polite copula ?? (desu).
Becomes a plot point in the first game — one of the Prinnies is special, and the cast calls her on it because she doesn't use "dood". So she tries, and sounds extremely awkward. The fact that she's pink (whereas every other Prinny is very, very blue) isn't brought up.
Pocket Kingdom: Own the World, one of the very few good-by-consensus games for the N-Gage platform, is intentionally filled with this, as it attempts to mirror an actual PC MMORPG. Players buy and upgrade their weapons with "loot", and losing characters are "owned", rather than simply being defeated.
Xigbar of Organization XIII talks like a Surfer Dude, though in his case it's more his personal quirk than anything.
Phoenix Wright: Ace Attorney: Justice For All: "That monkey doesn't fake the funk on a nasty dunk." To say nothing of Sal Manella, the fat, geeky TV director in the first game who largely communicates in 1337-speak when agitated, prompting your sidekick to ask "What are 'suck sores'?"
Zelos from Tales of Symphonia is a milder version of this; he doesn't speak in it constantly, but when he does, it's painful.
At one point in Tales of Legendia, an Oresoren acts as the translator for a huge, toothy monster whom his people reverently refer to as a "Mighty One". He speaks its words like a Surfer Dude, hilariously deepening his voice more then a few octaves as he does so. Some members of the party naturally question wheher it's actually talking that way, to which he insists that it is.
Parodied in Mario & Luigi: Partners in Time, where the plumbers encounter a Hammer Bros. duo mind-controlled by the bad guys via special helmets that talk 1337-speak. After they're freed from the helmets, they wonder who'd talk like that.
Parodied, again, in Super Paper Mario in the third chapter — which is usually referred to as the 'nerd' chapter. The (accidental) villain of the chapter is Francis, a "high-technicaaaal" nerd who abducts Ninja Butterfly Tippi thinking it's a rare insect, with no worse intent than to take photos of her to show off his new camera.
Super Mario World uses this for the Special World level names, which even for the time were likely painfully outdated to the point of sounding ridiculous. Groovy, Awesome and Funky are somewhat okay level names, but 'Way Cool' and 'Outrageous' sound like Narm, and Gnarly and Mondo are about as outdated sounding as possibly imaginable. As for Tubular, apart from being That One Level, the name isn't exactly slang most people on the planet would have even heard of, and trying to use that in normal conversation would merely gain a lot of odd looks from others.
The latter catchphrase results in a hilarious Ho Yay moment.
On the other end of the spectrum, we have Sho Minamimoto, who mixes mathematical phrases in with his dialog.
Sho:You fractals have no future! QED. Class is dismissed!
However, for the most part, the game uses modern slang, and properly, at that. The sheer volume of slang makes some conversations odd to listen to, though.
In City of Heroes, an officer representing Nemesis (a villain who's been around since the 1800s) tells a member of the cybernetic punk Freakshow: "I assure you, my good man, Nemesis is most definitely 'down with the street'. Word up, my homie, as it were."
Lampshade Hanging in Mega Man Star Force: When Geo travels to the AMAKEN compound, he meets a girl named 'Chatty Ditz' who's, like, totally having trouble, like, sending an e-mail to her friend, y'know? When Omega-Xis asks why she talks like that, Geo remarks that "it's some sort of dialect people used 200 years ago", to which Mega responds: "I'm not sure whether this means human language has reached its high point or its low point".
You encounter another largely incoherent speaker of fluent This-Trope in the Echo Ridge vending machine, who throws around a lot of words related to coolness that were obsolete in 2000, let alone 220X.
Also, parodied in Mega Man Star Force 3. One of the noise areas is inhabited by a corrupted wave being, which has a vocabulary that mostly revolves around one word, much to the confusion (and amusement) of the player.
In Command & Conquer: Red Alert 3, the Japanese Commander Kenji will taunt you during skirmish matches with phrases like "Hey look! A noob in training." or "What? This wet nose scrub beat me?".
Considering that this takes place (presumably) decades earlier in its timeline, this is probably still cutting-edge slang in-universe.
In Tony Hawk's UnderGround 2, slimy movie writer/director/producer Nigel Beaverhausen talks in labored, outdated slang to the skating teams. Mainly to get the point across that he's a total square whose attempts to relate to them are just condescending.
In the previous game there was a Russian man who constantly spouted slang and apparently learned English from "American hip-hop videos, my man!"
Downhill Jam parodied this with Gunnar, one of the fictional playable skaters in the game. He absolutely mutilates hip-hop slang in a way that would have been cringe-worthy...if he wasn't an Arnold Schwarzenegger Expy.
In the English localization of Xenogears, Hammer the Supplier says, "Master, sir, did you just see my MAD SKILLZ!?"
Griffith "Griff" Simmons' speech in SSX 3 was... painful. And, indeed, he does say "TOTALLY RAD!" as he's hitting a particularly awesome trick.
Not to mention Mac Frasier's terrible street lingo, which aged pretty pathetically. Yeah, Mac, we can say "bling bling." But nobody has wanted to since 2004.
Bad Dudes: "Are you a bad enough dude to rescue the president?"
Mr. Ekoda in Persona 3 attempts this in his first lesson to attempt to get students interested in his subject.
One of the villains in Sly 2: Band Of Thieves is Dmitri, a French Jive Turkey lounge lizard who learned all of his English from American music videos, and communicates in a mish-mash of outdated slang. Eventually lampshaded after Dmitri delivers a convoluted threat right before the boss fight with him, and Sly retorts with "I have no idea what you're saying. And your suit sucks!"
Suikoden V features Lu, a hyper teenage girl who uses 1337-speak and emoticons in her dialogue.
Sprung features constant overuse of baffling slang. It's hard to tell if it's earnest or ironic. Possibly both.
Take a shot every time you see or hear the word "extreme" in NBA Jam Extreme. By the time you're halfway into playing the Vancouver Grizzlies, the first team you face in Arcade mode, you will be too drunk to continue. At the very least, they spell it properly.
As can be expected from the title, the Cosmic Fantasy franchise used the word "Cosmic" on a lot of things, notably the Cosmic Pirates and Cosmic Hunters.
Data East USA localized Kaiketsu Yanchamaru for the arcade/NES as Kid Niki Radical Ninja, though the "rad" makeover affected little more than the game's title and the protagonist's hairdo.
Toejam And Earl. It would be hard to find someone in Real Life who uses the games' peculiar blend of slang without irony.
Borderlands 2 DLC Mr. Torgue's Campaign of Carnage has Flyboy, a 16-year old pilot who's constantly taunting his foes with some kind of weird slang that mixes Leet Lingo with Final Fantasy references with such terms as "Tough Tidus" or calling his opponents "Hojos".