It's totally possible to use the second language as well as native language. I am the example. It really depends on how much exposure a person has with the language. If you're forced to use it to express yourself all the time, if you read and write a lot, English would become the primary language for you. I was trained to use English as a formal language and also as a colloquial language. I've been at both formal and casual occasions, attending multiple schools and doing different jobs, meeting all kinds of people. Eventually, my brain is thinking in English only. Most people coming from China don't have the opportunity to use English as the only way for expression. And they don't get to work with different walks of people. I've been at the military, retail, fast food, security and more jobs than most of you guys ever have done.