Many people assume that you have to have grown up speaking more than one language to become an interpreter. While there are many interpreters who do have more than one native language, the majority of conference interpreters do not. As a matter of fact, growing up in a multilingual environment does not necessarily make you prime interpreter material.
Multilinguals routinely switch languages depending on the situation they are in, but many never have to establish a connection between their languages. On the contrary - their brains are actually used to suppressing the language(s) not spoken at a particular time to allow for the language of the current conversation to be produced fluently. So when they hear something in one of their languages, their brain does not automatically present them with the equivalent in their other languages. However, this is exactly what you want your brain to do when you are an interpreter. And you want this process to happen as quickly as possible, since any searching for the right word will slow you down and will make interpreting feel like a huge and exhausting effort. You want your brain to be wired for interpreting so that everything you hear pops up in the other language immediately.
When you only have one native language, any foreign language you add to your repertoire is automatically connected to your native language as you learn the new words one by one. “Car” gets connected to “Auto” when you are learning German, or to “voiture” when it is French you want to master. The new language is absorbed by your brain on the basis of your native language and cannot be disconnected from it. Your brain gets used to producing these pairs at lightning speed. Non-multilinguals with a knack for foreign languages can therefore become excellent interpreters. As can multilinguals who are used to connecting their languages, of course.
The good news is that the human brain is very flexible and can be trained to establish new connections all the time. This is what interpreters wanting to add another working language need to do. Once they have mastered the new language they will have to practice interpreting from it to increase the speed at which their brain presents them with the respective words in the target language. By learning the language they have established a railroad track between the words, as it were. They now need to gather experience in order to turn the track into a high-speed line.