I've been mulling over this a few days because it caught my eye for some reason.
I agree with you to some extent that cultural norms do affect behavior. However, I feel that culture doesn't effect introversion and extroversion, only the way some perceive what it is (Gah, that's a horrible way to say it). According to Jung, introversion and extroversion is determined by whether we are energized by our inside world or by our external world.
I think by saying that people mistyping INTX's in Latin countries for ENXX's is more due to people confusing introversion for shyness and extroversion for sociability. You can be an introvert and still be sociable and vice versa. While culture may change the way introverts act and the way society perceives them, they will still prefer their inner world over the outside world... you just may not be able to tell.
Of course, this isn't saying that an introvert can't change and become more extroverted, it's just saying that people confuse these definitions and make hasty judgments based off of them.
I need to mull over the T v. F part before I can write anymore about this...