I'm a Finn, and I used to believe that everyone in the world understands Finnish. Also when, say, two Englishmen would talk English to each other, they actually hear it as Finnish.
A childhood belief about speaking

others believed things like this
- I used to believe that people who don’t understand English can’t talk.— American
- I used to believe that everybody spoke English in the worldAnon
- I used to believe that the whole world spoke the same language- English.Anon
- I used to think everyone in the world knew and thought in English, but choose to speak different languages. Anon
- Olivia Newton John did sing let me hear your budgie talk! didn't she— J Scull
- I used to think that even people who spoke something other than English, thought in English— Peaches