Yup, Briar's main language is English (a mix between Unovan and Galarian accents after living in Galar for so long LMAO), and she also speaks French fluently due to living in Kalos during her early twenties. And lastly she speaks a little Spanish because it's her mother's first language and also because she traveled to Paldea for a while to study engineering :3c She's not 100% fluent because her mother spoke English at home and her time in Paldea was short, but she knows enough to survive

And yeah I guess her preferred language is English because she lives in Galar now LMAO But since all her academic studies were in either Kalos or Paldea, when it comes to science she ends up mixing French and Spanish words when she talks, at least when it comes to specific terminology that she learnt in another language. But generally she speaks only English LOL

Elio's a polyglot, and not by choice. Though he's capable of learning other languages easily due to his heightened intelligence via his Aura training - which was taken even further after becoming Ziodyne's new partner via expanding both his imagination and mental capacity, he notes that it would be a more enjoyable experience if it wasn't metaphorically beaten into him.

Learning multiple languages is part of the former "Argentium Perfectionism" mentality - more specifically, part of the "Being able to hold conversations well with others so that they won't think you're an uneducated rube who's somehow related to an important political figure", as perpetually bombarded in by his older siblings. It was an important skill when they would inevitably go out into the world via traveling or studying abroad. Not to mention dealing with dignitaries, government officials, and businessmen and women alike at important meetings and functions requiring their presence. It was especially essential when Elio and his younger siblings attended T.U.A. - an internationally-renowned academy celebrated as a literal melting pot of various people and cultures from across the globe; the younger Argentium set would naturally encounter other multilinguistic students and faculty from similar backgrounds or upbringings and said siblings would be drawn into conversations happening in different languages.

Elio doesn't put a number on how many languages he and his siblings are capable of speaking, or which ones each of them is fluent in for that matter, but he notes "...It's enough to get by without issue".