For what it's worth, ANA has pre-printed exit row instructions in Japanese and English which they show to any exit row occupants upon taking their seats. Their rule is that you must be able to understand either of the two languages. However, as recently as a few years ago there were stories of domestic agents denying exit row seats to foreigners at check-in because of expected language difficulties, even when said foreigners spoke Japanese fairly well.

While they *should* take the requirements seriously, I could easily imagine a slippery slope forming if they were to consciously clamp down on exit row requirements. At some point a clueless FA would force an English-fluent "foreign-looking" passenger to change seats, and then the airline would face a lot of bad publicity and perhaps even be sued. It's a bigger PR risk overall than the safety risk of an exit row passenger not being able to figure things out in an emergency. But yeah, that couple you saw probably should have been re-seated, or at least questioned further.