It's way easier to get over a cultural gap.

I've been in both situations, and it was a thousand times easier to work out issues with someone from the other side of the planet from a totally different culture and language, but who had a similar level of education and mental capacity than it was to work things out with someone from my own city and society but who was dumber than a fence post, irrational, and poorly educated to boot.

Studies about this have been done. In order for people to be compatible in a work situation they have to be within 20 IQ points of each other, from boss to workers and back again.

In order to be compatible neighbors and friends, they have to be within 10-15 IQ points for neighbors, and 5-10 IQ points for friends...

And in order to a be a mate or a spouse, they are always inside 5 IQ points of each other, regardless of how different their upbringing, culture, or native religion, but if they *are* inside 5 IQ point, and they want to be together, they don't care about upbringing, culture, or native religion... they just get over it and figure out how to deal with each other.

Notice how the issue is not how smart or dumb the couple are, but how closely matched they are. If they're slow witted and born with different mother tongues, bloody-hell they'll work out sign language chipped in rock if they want to be together.

But if they are too far apart on cognitive levels of mental speed and knowledge, it doesn't matter how similarly identical their culture and background and language and religion are... they won't get along for very long.