Does The Treaty Apply?
The Treaty says (in article VII not Article VI):
Each State Party to the Treaty that launches or procures the launching
of an object into outer space, including the moon and other celestial
bodies, and each State Party from whose territory or facility an
object is launched, is internationally liable for damage to another
State Party to the Treaty or to its natural . . . persons by such
object or its component parts on the Earth, in the air or in outer
space, including the moon and other celestial bodies.
The context of the referenced language is as follows, which matters because word don't mean the same thing every time that they are used.
This treaty is ambiguous, in the case of a "natural person" (i.e. a human being) injured by space junk who has some meaningful relationship to the member state other than being a citizen or national of the member state.
The term is not defined in the treaty itself, although it does reference several prior Outer Space related treaties in its preamble whose language and case law, if any, might be persuasive.
For example, suppose that someone from Croatia is on a NASA space ship when it is hit by space junk and that person is injured. I think that there are fair arguments for the Croatian passenger being a U.S. person (especially if the Croatian is a paid employee or contractor of NASA) and there are also arguments against that position.
Likewise, I think that there are legitimate arguments both ways if the Croatian is in U.S. territory when struck and killed by a space toilet seat falling to Earth (the premise of the TV series "Dead Like Me", minus the Croatian part). Immigration status (e.g. the existence of a visa, or citizenship or nationality) might be a factor, employment could be a factor, and other things could be a factor (e.g. if the immigrant were in custody, the U.S. has a legal duty to take steps to assure of the safety of people who are in custody regardless of immigration status, although this varies based on the nature of the custody in question). The existence of a duty on he part of a member state might cause that person to be one of the member "State's natural persons" even in the absence of citizenship or nationality.
In general, if the member state might have financial or legal obligations to a person if it did it, that person might have a remedy against other member states under the treaty, on the the theory that the goal is to make the state whole, rather than just its citizens.
To my knowledge, this issue has never been litigated, and it would be appropriate under the circumstances to look at the drafting history of the statute and a wide variety of other statutory/treaty interpretation approaches such as looking at how any treaty used as a model for this one was interpreted. I would also look at the several non-English language versions of the statute, in which the language might be less ambiguous than it is in the English language version, as all versions are equally authoritative.
What If The Treaty Doesn't Apply?
This said, even if the treaty doesn't expressly apply, general principles of tort law and governmental liability might lead to liability to our Croatian in any case on that basis. Nothing in the treaty says that people not governed by it can't or shouldn't be compensated.
Under general principles of tort law, in the U.S., some injuries are compensable only in cases of negligence, while others are compensable on a strict liability basis (e.g. ultra-hazardous activities and product liability).
The existence of the treaty might persuade a U.K. court facing the question of first impression of whether common law tort liability imposes liability on a strict liability basis, a negligence basis, or perhaps, in between, on a res ipsa loquitor basis (where negligence is presumed absence convincing proof to the contrary on the part of the party who caused the injury), impose a more lenient standard of proof on the injured individual or their next of kin in a lawsuit or claim action against the U.K. for money damages, on that theory that the treaty was merely meant to restate the general tort law rule that should apply in these circumstances.
If I Were The Judge
If I were a trial court judge faced with this case, I would make as detailed findings of fact as possible, as it is hard to know what an appellate court might find to be material.
Then, I would probably make a provisional ruling on whether the treaty applied, but would provide a contingent holding regarding the remedy if it did apply (if I held that it did not), noting that this is a matter of first impression, and would likewise provide an alternative hold regarding the right to relief and remedy if it did not apply (if I held that the treaty did apply).
If the outcomes were the same either way, the likelihood of an appeal would be reduced.
I would say that the holding that the treaty does not apply, but that there is tort liability anyway, is probably the most likely result in most of these fact patterns involving a Croatian with a non-citizenship/national relationship to the U.S., but I don't say that with any confidence in this uncharted territory.