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