Regulus
Regulus was a polyglot.
He grew up speaking English and French and was tutored in Latin and Greek. He used learning languages as a form of escapism, and to prove to himself that he was talented. While Sirius only learnt the most important phrases of Latin, Regulus spent his free time becoming fluent, even though it's a dead language. When Sirius came back from his first term at Hogwarts, he wouldn't shut up about his new best friend, James. He sung his praises constantly, telling Regulus the smallest details of his life, and the one that stuck with Regulus was that James spoke Hindi at home with his parents, his mother especially. Regulus decided to learn Hindi to prove to Sirius that he could do anything that James did, but when he was finally confident in his ability to speak it fluently, he was too ashamed of his own jealousy to actually show Sirius.
During his Hogwarts years, Regulus learnt Italian, as it was similar to French and came easily to him, and attempted German and Russian in his free time. He preferred languages that one of his friends spoke, so that he could learn pronunciation better, and when he and Sirius reconciled and he learnt that Remus spoke Welsh, he was determined to learn that as well.
When Regulus ran away to the Potters, James and his parents would sometimes discuss Regulus in front of him in Hindi, thinking he couldn't understand. Sirius knew several phrases after living with them for so long, and would join in when he could. Usually, the conversations were about what they were to do with Regulus. One day, Regulus gets mad when Effie suggests that he see a therapist, and when he gets aggressive, which leads to Sirius coming to Effie's defence and a shouting match ensues. When they calm down, Regulus is reading a book on the sofa, and Monty suggests in Hindi that they bring Regulus to a therapist without telling him, just to see how he likes it. The others are discussing it, how it would be achieved, is it ethical, etc.
They are wondering if this is the best idea when Regulus says in perfect Hindi, that perhaps they could just talk to him about why he doesn't want to go to therapy. James and his parents turn to Sirius in shock, asking when he got so fluent in Hindi, as his speech is often stilted, and he looks at them, confused. He tells them that he didn't say anything, and Regulus shuts his book with a snap, and leaves the room dramatically, without saying a word.