It is quite important than words and symbols used by wizard are not his vernacular. They do not hold like gnostic value in doing magic, they do not represent some angelic language or smth like in real-world occultism - they are foci for wizards mind.
If Carlos is from L.A. he could use some native Mexican symbolism for his spells.
He could have, but why would he when he goes to the effort of distinguishing himself from Latin American roots? That's why I say it gives us a week implication that his master identified with that language and could be part of a native bloodline.
Justin used, and taught Harry and Elaine to use, Egyptian. Is that the standard practice? Maybe. We don't have much evidence on what are traditional master-apprentice techniques and relationships. Harry, being Harry, decided to go with faux-Latin. Justin let him because it would serve to further isolate him from the Council. Or that's my guess as to why. Harry let Molly use Japanese and didn't teach her to use Latin for the opposite reason. Again, that's my guess.
He had bad study habits from the get go, he slept through Latin in high school.
He learned Latin from a correspondence course. That's why he keeps thinking "stupid correspondence course" in the earlier books. Harry did bad in Spanish in high school. Harry actually had good study habits until him and Elaine became sexual. Harry comments that the schoolwork was easy.
I remember an old Woj about Harry doesn't do potions anymore cause they're for people who really don't know what they're doing.
I've heard several where Jim talks about how they're a crutch for writers. I've heard another where he said potion scenes are boring or he doesn't enjoy writing them or something. Another one said that the pacing was too fast for Harry to have time to make potions.
So it makes sense for Harry to use less potions and maybe use some we've seen before.
Honestly, I'd prefer it if Harry made some potions at the beginning of a book every now and then that he expected he might need and didn't always end up with an opportunity to use them. It would show him being prepared, like he always goes on about, and be realistic in that sometimes we prepare for risks that never materialize. Like how I wear a seatbelt every time I get in a car, but have only needed one
maybe once.