If a baby was born on the ISS, what would its nationality be? Many countries grant citizenship to any baby born within their territory, but the ISS is not national territory ... or is it? If a spacecraft belonged to one nation, citizenship would likely be extended by that nation, as happens on a ship at sea.
If a baby were born on the ISS, it would (probably) generally be treated the same way that it is handled here on Earth if babies are born on foreign soil. This means, it will be determined primarily on the citizenship of the parents.
Specifically, there are two primary legal contexts here: "jus soli" (right of the soil) and "jus sanguinis" (right of blood). Jus soli is the reason people born on USA-territory are automatically US-citizens, however jus sanguinis is more common worldwide, where the citizenship of the child is determined by those of the parent, regardless of birth-location.
For example, if a child that was the result of two German citizens was born on the ISS, it would be a German citizen as Germany generally uses a jus sanguinis system for their citizenship.
With US citizens it's a bit more complicated and fuzzy, but generally a child born to two US citizens will also automatically be a US citizen, regardless of where the birth takes place, provided that at least one of the parents has had a residency in the USA in the past.
Addendum: The ISS is a bit special because it is not "truly" international, but rather, each country maintains legal jurisdiction and authority over their sections. In the US sections, US law applies, in the Russian section, Russian law applies, etc. Despite this, the individual sections are explicitly not counted as "sovereign territory" which is the requirement for jus soli [source].
The baby might have been born in space, but its parents, i.e. the ISS astronauts, were Homo Sapiens who originally came from Earth.
So, your "baby" is about as "alien" as a panda i.e. your baby is not an "alien" even if you decided to paint him up green and attach antennas to his head.
Unless your setting in thousands of years in the future where "space-humans" and "Earth-humans" have diverged into two different species, chances are, your "baby" is about as alien as a rock.