It’s harder to turn a language off than on. When a bilingual person changes between languages, the most effort lies in deactivating one language, not picking up the other.
In research published in Proceedings of the National Academy of Sciences in September 2018, United States researchers studied bilingual people fluent in English and American Sign Language who often used both at once. They were able to leave one language on while turning off or turning on the other—and engaging the other language showed no more activity in cognitive control areas than not switching languages at all. Using two languages, the study concluded, is no more difficult for the brain than using one.