السلام عليكم ورحمة الله وبركاته Considering the obligation to abide by the laws of the land, would it be haram for a Muslim man living in a country (which legally doesn't allow or permit polygamous marriages) to take a second wife through an Islamic nikkah?

The nikāḥ itself is valid when its conditions are met (the walī, two witnesses, and announcement), even if it is not registered. Also these lands they consider these to be affairs or so, which for you does not make a difference as marriage for you is what is Islamically considered.