Erin Ptah

I'm calling the new girl Bonita, so Lyra's two closest friends can be Bon and Mal. She's from High Brazil, because the place comes up a couple times in passing, and this would be a good setup to give some actual worldbuilding about what it's like.

We need more non-mammal daemon rep, so Bonita's is a caiman lizard: semi-aquatic, and native to South America. His name is Potameides. Like Pantalaimon, it's Greek (it's a kind of river nymph)...and it means their nicknames can be Pot and Pan.

Contrasts. Book Lyra has short blonde hair, so Bonita's is long and dark. Red-brown mammal daemon vs. green reptile daemon. Lyra goes by Silvertongue, so Bonita's family name is Eldorado, "the golden one."

All the random things Pan accuses Lyra of in TBoD, that we never see on-page? Bonita actually does them. Dismisses and ignores her daemon. Sneers at art and beauty, saying they're irrational. Finds out about Lyra's field of study, and scoffs that the alethiometer symbols are so open-ended, you can read anything you want into them.

On the other hand...Bonita is very into this book about a daemonless hero who kills God. Lyra's first great love was a daemonless hero who killed God! She's gone for so many years without a single person in her life who isn't dismissive and/or scared to discuss that subject.

Turns out, Bonita is the only student besides Lyra who isn't intimidated by the Magisterium in general. And Lyra is the only student who isn't intimidated by Bonita. So they end up forming a weird-but-genuine bond.

Mostly, don't do it.

Translation Convention is your friend. Write in the language you're comfortably fluent in. When that's not the language the characters are using, just explain that in the narration:

"Here's an example of how to do it," said Duchamp in French.

Translation Punctuation is your other friend. Especially if you need to help your readers keep up with something complicated:

"Say, if the characters tend to [switch languages in the middle of a sentence]," he added.

If you try to write too much of the dialogue in a second language, it's an extra hurdle for readers who don't know the language. Especially if they have other accessibility issues too. (Somebody using a screen readers, for instance, can't easily skip to the endnotes for a translation and then scroll back to the original line.)

And if you're not fluent in the second language, readers who are fluent are probably just going to be annoyed! Because they'll notice "this is too stiff to sound natural, this is not the jargon he would use, this is the wrong familiarity level for the person he's talking to" -- all the subtle ways of being Weird and Out-Of-Character that you, as a non-speaker, won't even realize you're putting in.

When I put non-English dialogue in my fics, a lot of it is single words or short expressions, which softens the "hurdle for readers" problem. Here's a genuine bit of Frenchie's dialogue that shows up in my fic:

"Desolé, mademoiselle." Duchamp raises his own coffee to her in a friendly toast. "I am afraid my making-out proclivities run to 'hot guys' exclusivement."

And here's some Jeanne-Marie, from the same chapter:

"No more of ze Dust!" snaps the Canadian ex-Widow. "Je n'ai pas besoin d'aide. I am in my right mind now! More zan she ever is!"

The French parts have translations in the endnotes. But it's not like you'll be completely thrown out of the fic if you don't already know what they mean. You can pick up the overall idea of "Duchamp is politely rejecting a woman flirting" or "Jeanne-Marie is angrily refusing help" from context.

Short phrases also make it easier to find the in-character nuance you need, not just the literal meaning. There's existing info online about "how formal/rude/personal/slang-y/etc is this?" for common expressions, in a way that there isn't for full lines of fic-specific text.

I check WordReference a lot for Spanish and French -- especially in the forums, where users will have thoughtful discussions about "can I use term X in context Y, or are the vibes off?"

One of the perks of fandom is, if you reach out for help, you can explain it in terms of the canon you're all already fans of. At some point I directly asked in a Moon Knight Discord, "what's a good derisive phrase Jake could use to refer to Elias, that means something like enabler or bystander?" A couple Spanish-speaking fans suggested buena para nada, and I've been putting that in fics ever since.

So, yeah, that's my advice about "naturally working other languages into your writing."

TL;DR Do it very little! Focus your research on getting the little bits right.