uh i disagree, with all my years of medical training

there's a huge neurological difference between one's first language and more recently acquired language. (do you imagine sun thinks in english? of course not.) english to you and me is largely instinctive, or close to it. it's still possible to lose the ability to talk, but a language you've spoken for five years would be FAR from that. there's a world of difference in the brain between knowledge so deeply ingrained as your first language and a language learnt in the classroom or from a tutor. think of one as remembering how to ride a bike and the other as remembering all the steps to a complicated, multi-stage exercise. there are neurologically relevant differences there, whether you believe it or not.

tl;dr it is plausible enough to fit in a sci-fi tv show; whether you like it or not as a plot device is another damn matter