The the rules about whether you can instruct at all are the rules of the country you are in, not the FAA's.
Under the FAA's logging rules,
A certificated flight instructor may log pilot in command flight time for all flight time while serving as the authorized instructor in an operation if the instructor is rated to act as pilot in command of that aircraft.
An "authorized instructor" is (among other things)
A person who holds a flight instructor certificate issued under part 61 of this chapter and is in compliance with § 61.197, when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate;
I don't see a clear answer but I'm going to take a liberal view. Nothing in there says "in the US." 61.51(j) says in part:
Aircraft requirements for logging flight time. For a person to log flight time, the time must be acquired in an aircraft that is identified as an aircraft under § 61.5(b), and is--
***
(2) An aircraft of foreign registry with an airworthiness certificate that is approved by the aviation authority of a foreign country that is a Member State to the Convention on International Civil Aviation Organization;
I don't think it's a big jump from there to say that if the aircraft is not US registered, it depends on the rules of the "host" country. If you are authorized to act as an instructor under the host country's rules, I think you are fine logging PIC as an authorized instructor.
Whether the FAA would agree with me is another matter.