Speakers of English may say, "What is his native tongue?" meaning language, and those who spoke Latin used lingua with the same figurative application. A linguist, then, is thought to be someone who speaks several languages fluently and is also known as a polyglot, literally, "many tongues".
Usually, when this person was bilingual; the next step probably was to be trilingual or even multilingual. From these stages of language development, we know that this person has learned diction, wording styles, idioms, phrasing, and vocabulary; as well as slang, jargon, and dialect because that is what linguistics is all about.