Let’s say there is this dissertation:
The impact of interest rate on economic growth in country X
Can I write my dissertation about:
The impact of interest rate on economic growth in country Y
Let’s say there is this dissertation:
The impact of interest rate on economic growth in country X
Can I write my dissertation about:
The impact of interest rate on economic growth in country Y
It is not (necessarily) plagiarism (unless you actually copy parts, or merely rewrite the previous thesis), but it would make a pretty boring thesis. A PhD should add something new to a field; investigating the same thing in a different group/country would probably not be enough. Therefore, you can, probably, not write your thesis about it (i.e. the thesis committee/advisor/... would probably not accept it as being sufficient for a PhD thesis).
Unless, that is, you add something interesting. One possibility could be (though keep in mind that I am in a very different field) to say that:
According to previous work, the effect of foo is bar, as shown by studies in Country X. Country X is a dictatorship/christian/... country. There are reasons A, B and C to suggest that foo⇒bar has been observed because of Country X being a ... country. According to our theory, in a democratic/pastafarian/... country the effect of foo should be buu, not bar. We test our theory in Country Y, which has such a system. Furthermore we show that the effect foo depends on [country property] by doing some more studies.