Starring: Brittany Murphy, Pamela Adlon, Mike Judge, Kathy Najimy, Johnny Hardwick
Returning Japanese (1) - S6-E21
Plot hole: In this episode, Hank meets his paternal half brother. They look alike. However, we saw in an earlier episode that Hank's facial structure takes after his mother, not his father. Since they are paternal half brothers, then the Japanese brother cannot look like Hank since he has a different mother.
Hank: Calm down, Boomhauer! I can't understand a word you're saying.
Question: How can Dale be so oblivious to the fact that Joseph isn't his biological son or that Nancy cheated on him with John Redcorn considering in one episode Dale actually caught them both in bed together.
Answer: That's the shtick. Everyone in town knows that Dale's wife is seeing John behind his back and that Joseph is obviously really John's son, but Dale is completely oblivious (he's not that smart of a man). In one episode, Dale even says he's not homophobic because 'John Redcorn is gay, and he's one of my closest friends.' Of course there is a fan theory that Dale is completely aware of what's going on but chooses to ignore it out of fear of his marriage ending.
Also, if Dale *does* know about the affair, what choice does he have anyway? Explain to everyone he meets that Nancy is his wife, and Joseph is her son from her boyfriend? If he wants to stay with Nancy, then raising Joseph as his own child is slightly less awkward. Especially while Joseph is younger. I know of one man who did this. It was obvious that his wife had an affair, and "his" younger daughter had been fathered by someone of a different race. An "elephant in the room" situation.
Answer: In addition to Brian Katcher's response: it's supposed to be a joke that Dale is always suspicious about conspiracy theories, mysteries, etc, but he is "clueless" about his own wife having an affair. Of course, as Brian wrote, many fans think that Dale is pretending to be clueless because he doesn't want Nancy to leave him.