I didn't exactly believe it but I suspected that my language (German) was an invention by my parents and that everyone I ever met were paid by them for talking this artificial language.
I'm still not entirely sure.
A childhood belief about foreign languages

others believed things like this
- I used to believe that translating from language into language was only a matter of changing letters ! :) — EwS
- I used to believe that if I spoke with whatever accent a language was in, I was speaking that language...— Renae
- I used to think that my aniamls were going to talk to me but i still think that they have their own language.— Jane Rom
- I used to believe opera singing was a language all its own.— Rachel
- i thought grown ups read in a different language!Anon
- I used to believe that the whole world spoke the same language- English.Anon