Langley Diner is where Stan confronts Roger when he finds he has become a crooked cop in "Cops and Roger", telling him that Roger is acting dishonorably. Roger responds that he now feels like he has all the power he lacked before and likes it. Roger storms out, but not before ordering a busboy to put a gallon jar of mayonnaise in his car and takes a customer's pie, using it to leave twin butt-prints in the diner's doors.
When Klaus is forced to admit to the guys that he lied about getting married just so they would spend a weekend with him in "Men II Boyz", they ditch him to hang out at the Langley Diner. However, he catches up and offers them a round of milk, but when he is unable to pay for it, he is put to work washing dishes comically, which annoys Stan into paying for the milk after all. After confronting him about his lies, he slowly comes clean. But when they think he's lying about his breakup with Shoshanna who kept his Instant Pot, he insists he's telling the truth, so they decide to help him recover it.
Steve briefly sees the diner while having an out-of-body experience in "The Wondercabinet".