AIUI, a federal U.S. prosecutor can subpoena any witness or victim in the United States, regardless of nationality - and/or a U.S. citizen anywhere in the world - but it does not extend beyond that (if, for instance, they want a non-American, in a foreign nation, to testify and provide evidence to be used in prosecution in U.S. court, that would have to be done on an entirely “voluntary” basis and isn’t something the American prosecutors could actually compel them to do?)
Strictly speaking it’s the court that subpoenas a witness, on the application of a party to litigation. Which is not a nitpick; it’s relevant to your question because it’s the source for the answer to your question. A court can subpoena anyone who is subject to the jurisidction of the court. Which is to say, for a US federal court, anyone in the United States, and anyone outside the United States who can be made amenable to the jurisdiction of the court.
I suppose in strict theory there is nothing to stop a court issuing a subpoena to someone who has no connection at all with the United States. But would there be any point? The person would be free to ignore the subpoena and there would be no mechanism for enforcing it.
Countries usually have have bilateral Mutual Legal Assistance Treaties (“MLA Treaties”). These lay out the procedure and practice for cases like this. Typically, what happens is, the Court will issue process and a request, process is for the US Governmeny bodies to contact their foreign counterparts and submit the request for them to petition the competant Courts in that Country to execute the US Courts process.
Absent an MLA a request could still be made, but would be totally voluntary, ie no Treaty obligations.
So in essence, its like when you were a kid and your friends Mum wanted to ask you something. She’d call your mum for permission and or ask her to ask you.
Missed Edit:
In absence of a Treaty, the Court can issue a request, known as “Letters of Rogatary”.
The US State Department helpfully has a website with this information.