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.
I can read multiple of dead languages BUT I can never speak any other language but English. I can read (non-modern) Hebrew, Classical/Konie Greek, Aramaic, Latin and somewhat (not dead) German. Thank you Theology Studies the Nerds great subject to learning language and history.
I now work with 250 students in Head Start (3-5 years old) and about 20% speak mainly Spanish and I can speak to any 5 or 6 year old in Spanish fairly well, but adults I sound like a 5 year old. I try to talk with the staff where 50% speak fluent Spanish and well they always go back to English and I never progress. :(