So surely there's also plenty of room for bad, old-fashioned human-on-human racism.

Pratchett thought so, too. Jingo (1997) is a sharp rebuttal to, "Racism is not a problem on the Discworld."

Take the characterisation of 71-Hour Ahmed, bodyguard to the Klatchian Prince Khufarah. From Sam Vimes' perspective, Ahmed acts every bit the stereotype, compared to the honourable, educated Prince. The eye dialect and clunky Morporkian comes out in full force (👇):

> “If hyou changing your mind, offendi, I give hyou twenty-five camels, no problem,” he said, pulling a clove from his teeth. “May your hloins be full of fruit.”

> “I go, I hcome back,” he growled happily. “The Prince hsays the degree is Doctor of Sweet Fanny Adams. A hwizard wheeze, yes? Oh, how we are laughing.”

At the start, Ankh-Morpork and Klatch are in diplomatic dispute over what to do about the newly-emerged island of Leshp, smack-dab in the middle of their waters.

> “Constable Visit’s pretty brown,” said Nobby. “I never seen him run away. If there’s a chance of giving someone a religious pamphlet ole Washpot’s after them like a terrier.”

> “Ah, but Omnians are more like us,” said Colon. [...] "Just the same as us underneath.

Never underestimate anyone's capacity to discriminate against foreigners over the tiniest things - especially Colon, the median voter in a 'one man, one vote' city state (👇):

> "…Whereas we’re more civilized, see, and we got a lot more stuff around to count, so we invented numbers. It’s like…well, they say the Klatchians invented astronomy—”
“Al-tronomy,” said Nobby helpfully.

> “No, no…no, Nobby, I reckon they’d discovered esses by then, probably nicked ’em off’f us…anyway, they were bound to invent astronomy, ’cos there’s bugger all else for them to look at but the sky.

When the Prince's life hangs in the balance, after an assassination attempt, Ankh-Morpork mobilises for war. Everyone suspects Klatch of a false-flag - everyone, that is, except for Vimes. He suspects his own fellow Morporkians first.