I worked with developmentally disabled adults for over 12 years; our program participants all had IQs that measured at 70 or below and most had other issues as well (cerebral palsy, various syndromes like Down's, Williams, or Fragile X, low-functioning autism, being visually or hearing impaired, etc.).
The way D&D measures Intelligence honestly has absolutely no correlation to the way Intelligence works in real life. Same with every other RPG I've played that has an Int-type stat. In the real world, having a low IQ is correlated with actual physical changes in the brain, which cause other physical problems such as poor motor skills and a host of other physical and mental health issues, along with social issues as well (effectively, the people generally often unable to learn what was socially appropriate). To be realistic, any D&D PC with an Int of, say, 4 or 5 lower than the norm for the race should also have trouble walking and using their hands, or even be unable to do so. "Realistically," the PC other five scores should be low as well, since the brain really does control everything.
Obviously, this isn't the case in D&D, where you can play an Int 6 character with zero problems.
TBH, I think the examples you gave of how to play a low-Int character,
@ad_hoc, are not particularly good ones. Several of them (being unable to memorize facts, taking risky behavior, no attention span) are also indicative of disorders like ADHD, which has
nothing to do with intelligence. Neither lacking confidence nor being in a different culture indicate a lower intelligence, either--and those characters could still be
very intelligent, even if other people don't realize it.
Not to derail this into a thread about floating versus fixed ASIs again, but this is why I don't like fixed ASIs: because the stats are so divorced from reality that they don't actually
mean anything beyond getting a bonus or penalty in your rolls or a few other basic mechanics. You can play a character as an absolute genius who simply is really, really bad at doing anything that requires Int-based rolls. A few of the developmentally disabled adults I worked with had amazing skills of memorization.