The author is stating that once you live in a country that primarily speaks the language you are learning instead of your native language, then the language you are learning can become and feel as natural as the one you grew up speaking. Sometimes when people live in a country for three or four years they begin to reach the peak of their fluency in the new language and begin to make very minor mistakes in their born language. Edit: TL;DR without moving to the country there is only so far you can go with fluency -speak it every day -speak it naturally -dont translate things from your native language to the language you are learning to understand it and vice versa