It takes unusual circumstances for someone not to be a native speaker of a language. Normally, one has a mother tongue, the language of one or both of one’s parents. Unusual circumstances of another sort can produce the situation of a person speaking more than one language like a native. That is the situation in which Y-H-B has found himself all his life due to circumstances of birth and biography.
I was reminded of this conjuncture when eating lunch in a Japanese restaurant today in Beverly Hills. My habit is not to speak Japanese to waiters and waitresses in such restaurants so as not to shock them. Members of the Nipponese nation are still not entirely habituated to hearing native Japanese emanating from Caucasians, and in my case the temptation to épater le bourgeois is routinely resisted.
Speaking like a native also involves cultural paralinguistic patterns that one falls into effortlessly and almost obligatorily. It’s as if one took off one mask and put on another when speaking a different language. The Japanese cultural situation is full of behavioral peculiarities that are incumbent on one who speaks the language natively, including nods of the head, intakes of breath, ejaculations, etc. that are wholly absent from the speech situation attending American English. Withal, these features are ineluctable for someone who is a native speaker, and a trilingual one (like your Y-H-B) cannot escape them any more than a monolingual one can.
Switching to Russian, as Y-H-B had to for long stretches during a recent week-long stint at Eastern Washington University while conversing with one’s host––a man born and bred in Russia––entailed quite other sequelae. One’s mentality shifted from the habitual Anglo-American context to the poetically-inflected world of Pushkin and the Golden Age, including Krylov’s fables, which bodied forth a quite different personality.
Language is the instrument of thought, to be sure. It is also the determinant of one’s personality in many respects. A trilingual speaker must maneuver effortlessly between the worlds that a native knowledge of three languages adumbrates, and it is not always a task that a normal psyche can accommodate.