It gets worse. I'm bilingual since childhood. I mostly speak English. People can very obviously tell that the second language is not my native tongue. This

It gets worse. I'm bilingual since childhood. I mostly speak English. People can very obviously tell that the second language is not my native tongue. This is after 30 years of using it. It comes down to how much of your day is spent conversing in a language, I'd guess that if it's less than 20% it's a dead giveaway. Speaking to adults is completely different than speaking to children. The Pareto principle he cites is actually deceiving. Yes learning 20% of the vocab gives you 80% of the commonly used words. Now try and go to 90%. If I remember you have to double effort. So e.g. if 80% is 1500 words, 90% is 3000, 95% is 6000, 97% is 12000, etc. It's diminishing returns. The problem is to have adults conversations all the time you probably need a vocabulary of around 6000 words, depending on the context of the conversation. Ever hear what a foreigner sounds like when they speak broken English or English with a strong accent? That's exactly what you sound like when you're talking to someone else in their language. Like some complete retard. Mess up one single thing or say something unidiomatic and the show's over. If they feel their English is better they'll switch immediately, but just keep going and hopefully they'll switch back.