Maybe it’s the different translation, or maybe it’s that I’m seeing the book differently now, but the chapter on argot (thieves’ slang) no longer seems out of place at all. The plot comes to a dead stop, but no more than it does then we learn about, say, the July Revolution and Louis-Philippe. And there are a lot of scene fragments that are moody, atmospheric, and downright disturbing.
And thematically, it’s spot-on.
Everything in this book is about theme. Victor Hugo wants to show you how awful it is to be in society’s underclass, show you how society is complicit in keeping them down, show you that at least some of them are worth being uplifted, and convince you to at least sympathize and at most try to help.
The plot and characters are ways to get the theme across. But they’re not the only ways, and he wants to make his case by as many paths as he can. So he includes history, sociology, philosophy, religion, science, statistics, and now linguistics.
Underneath all the “OMG they’re bastardizing the glorious French language!” and “I know, how dare I write about vulgar language? Here’s why I have to do it” is a metaphor of slang — specifically criminals’ slang — as a symptom of intellectual deprivation, going along with the physical deprivation that the poor suffer. He links it to other languages (and therefore the broader world), he ties in the inhumane treatment of prisoners, and brings it all back to the idea that if you put more effort into enlightening people’s minds to begin with, you’ll have less trouble with them going bad. Education first — like the ABC society’s stated goals, and one of the key themes he lays out at the beginning of the novel.
It’s true that he makes a lot of the same points elsewhere in the book, but I think Hugo is trying to take a comprehensive approach, like a teacher whose students have multiple learning styles who tries to include something for each of them to grasp.