I'm not sure I agree with that. I grew up with German as my mother tongue, but with a strong English environmental influence (TV, strangers on street etc). Both taught as 1st language in school.
It starts out with "thinking in one language, speaking another", but as you improve in both it just becomes a blur - to the point where I can't tell you whether the thought I just had was in English or German. Sometimes I think the thought just "is" as an abstract concept language-less concept. Sort-of like a geometric shape isn't German or English.