You mean "Qunthk", I assume? Unless there's another Q-named language in the series?
This was very hard to find and took me until the last page of a Google search of "Jinx "Sage Blackwood" language" to even find the name.
The quote you're referring to is from the second book in the series:
Why do you have books in Qunthk? Can you actually read them?”
“Not exactly,” said Jinx. “Can you?”
“No. It’s famously impossible to learn, because the words are all stuck inside each other. What are all these pictures of bottles?”
“What do you mean, ‘stuck inside each other’?” said Jinx.
“That’s how you make a sentence in Qunthk. Instead of stringing the words one after another, you split the first word and put the second inside it, and split that and put the third inside, and split that—”
Jinx's Magic