The article mentions that French was his first foreign language. He picked up Russian to read his first calculus book, then English to do an Oxford maths degree

The article mentions that French was his first foreign language. He picked up Russian to read his first calculus book, then English to do an Oxford maths degree. And you're unlikely to be a doctoral student in philology and know only one foreign language, so he had undoubtedly learned at least one other language between the French and the Russian. People don't always realise just how scary serious students of linguistics are.