Is this based on theoretical reasoning or do you have anecdotal evidence for it? I'm genuinely curious because I don't know what to think on this issue.
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 l...
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.