Charlie is terrible with languages - he’d tried to pick up several over the years, especially when he lived in Eastern Europe, but to no avail.
WSL - Wizarding Sign Language - was adapted as a more reserved form of regular BSL by a deaf Ravenclaw who kept triggering accidental magic with the excited movements of her hands.
Sybil Trelawney was born speaking Ancient Greek. No one knows why. It was hardly the thing that scared her parents the most.
Ron has an unexpected knack for languages, and after the Parseltounge incident during the Battle of Hogwarts, Hermione encourages him to explore it. He picks up French and German without a hitch (although he can’t spell for shit in either), and learned Giant Language on a whim to make Ginny laugh. All three come in handy as an Auror.
Fleur takes great pride in being bilingual. She thinks about her history in French and her future in English.
Snape writes up all his formal calculations, experiments, and new Potions recipes in Latin. It narrows down the amount of people who can read it, and comes in useful to recite in his head while practicing Occulemency.
Hermione kept up her study of Ancient Runes after leaving Hogwarts - partly because she loved the way it made everything simple and clean, just tracing the power words hold, and partly because it made her indispensable at the Ministry. Most of the best Runologists of the generation had taken a side during the war that made them no longer welcome in polite company.