A training exercise. Somewhere I have a photo of a French aircraft operating on a US carrier. Or it might have been a emergency landing. Helo landings on on a differently flagged ship are not quite as eye catching but still not routine.
Quite a while ago a USN officer related how his ship had a urgent medical evacuation necessary. A Japanese self defense force helo was the only availalble. The US Naval officer related how he managed to convince the JSDF pilot that a helo landing aboard a destroyer, at night, in 10-20 knot winds, with rough sea state, very low overcast.... was in fact routine and no problem. The pilot with no previous experience or training at deck landings managed with a lot of coaching to safely set his aircraft down.