Most intelligent persons of the world

[MrBehemoth]

learned one language per day.

Not possible. To completely learn a language takes a lifetime. We're talking hundreds of thousands of words. French has a relatively small number of words, about 100,000. Assuming he slept for 8 hours and didn't stop learning words while he was awake, that's 2 words per second, leaving no time to master the grammar or pronunciation.

Maybe he could learn to repeat a few sentences and so seem to be conversant on a given topic.

It was later acknowledged, however, that some of the claims made were exaggerations, with a researcher stating "I have been researching the veracity of primary sources of various subjects for about twenty-eight years, and never before have I found a topic so satiated with lies, myths, half-truths, exaggerations, and other forms of misinformation as is in the history behind William Sidis"