It just follows from my model of the world, that the intelligence of a person correlates with their ability to react quickly, to make and understand jokes, to use a nuanced vocabulary, etc., and all these abilities are impaired when the person must struggle with the language or speech itself.
Of course there are also other things correlated with intelligence which don't depend on language skills. I'm not saying that an intelligent person will seem like a complete idiot just because they speak another language. It's more like a person with IQ 150 will seem like a person with IQ 120; and a person with IQ 120 may seem like a person with IQ 90. The difference would depend on their language skils.
I don't know if anyone tested this experimentally, but it should be easy. Take a few foreigners, give them some standardized English tests and IQ tests... then let them do some verbals tasks (e.g. tell a story) in front of the audience... then let the audience rank them according to their intelligence.
lt;dr -- Speaking slowly, making mistakes in difficult phrases, misunderstanding jokes... is evidence of low IQ. But it's also what foreigners do when speaking your language.
It's more like a person with IQ 150 will seem like a person with IQ 120; and a person with IQ 120 may seem like a person with IQ 90. The difference would depend on their language skills.
I am pretty sure that this doesn't happen. The reason is that when you speak a language so badly that you make those kinds of mistakes that make you appear stupid, you also have an accent - and that is a sign to the listener that they have to account for your being someone with potentially poor command of a foreign language.
This won't make the effect vanish completely, b...
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.