When my brother was about 5 years old, he came to me asking why people spoke other languages. He had just assumed everyone thought in English but translated it into other languages in their heads before saying things.
It was really tough to explain that many of these people didn't know any English, much less think English thoughts.
A childhood belief about foreign languages

others believed things like this
- At the age of four I believed I was soooo smart. I spoke TWO languages...English AND American!— Melinda
- I used to think that even people who spoke something other than English, thought in English— Peaches
- I used to think everyone in the world knew and thought in English, but choose to speak different languages. Anon
- I used to believe that, no matter what language other people spoke, that everyone thought in EnglishAnon
- I thought that people who spoke a different languages were aliens.— Phoebe
- I used to believe that anyone speaking a foreign language translated it into english in their head— galen