So when the Constitution speaks, it is the highest authority and trumps all others, including the unwritten so called natural law. I would point out that natural law is INFERIOR to the Constitution. When the Constitution speaks, natural law has no weight.
Article 1 Section 8 clause 4 enumerates Congress with all powers regarding the rules of naturalization and citizenship. That includes:
- when naturalization occurs (after one’s birth)
- who needs to be naturalized
- who does not need to be naturalized (citizen at birth, i.e. naturally born a citizen)
- the conditions by which a person qualifies for citizen ship such as birth on US soil (14th amendment) and any other conditions Congress desires.
With regards to your questions - other countries are sovereign in their own right and can grant citizenship how they so please. However, their laws have no impact on US laws with regards to US citizenship.
Additions might always be made to the citizenship of the United States in two ways: first, by birth, and second, by naturalization. This is apparent from the Constitution itself, for it provides that 'no person except a natural-born citizen, or a citizen of the United States at the time of the adoption of the Constitution, shall be eligible to the office of President,' and that Congress shall have power 'to establish a uniform rule of naturalization.' Thus new citizens may be born or they may be created by naturalization.
The Constitution does not, in words, say who shall be natural-born citizens. Resort must be had elsewhere to ascertain that. At common-law, with the nomenclature of which the framers of the Constitution were familiar, it was never doubted that all children born in a country of parents who were its citizens became themselves, upon their birth, citizens also. These were natives, or natural-born citizens, as distinguished from aliens or foreigners.