On the nature/nature aspect people are already predominantly nature. For the nurture component of the residual the early years are very formative. As an expat who has spent much more of my life outside my country of birth than in it, and knows many other expats who have done the same, from my observation our upbringing still dominates our behavior. It is also a selection criteria bias where expats are more likely to identify with other expats even when not from the same country or residing in the same country which is one of the reasons expats tend to form communities with other expats. The problem with self assessing behavior is that the same biases which determine behavior is used for assessing the behavior where much of what is considered merely normal is ignored leading to over-weighting the size of the unusual component.

But the Church hierarchy probably decided the young priest would be stationed in Peru, not the priest himself.

Sure, but the fact that he stayed there for many many years means that it suited him. If you want the change countries the Church has processes for that.