The fact that early languages, no matter how many there are, utilize the same streams implies that the brain doesn't have a native language. The brain can only reflect the fact that a set of neural circuits was built and activated for a certain period of time. Nor does the brain care if those neural circuits map onto things that the rest of the world calls languages or dialects. It really cares only about what activates those circuits. Thus, the brain patters that typify language use across skill levels can be mapped.Brain imaging technology monitors the intensity of oxygen use around the brain - higher oxygen use represents higher energy use by cells burning glucose. The deeply engrained language circuits will create dim MRI images, because they are working efficiently, requiring less glucose overall. More recently acquired languages, as well as those used less frequently, would make neural circuits shine more brightly, because they require more brain cells, thus more glucose.
Michael Erard, Babel No More: The Search for the World's Most Extraordinary Language Learners
brain, language, linguistics