Here’s a visualization of the birth rates of Japan, Poland and Germany against those of South Korea:
https://georank.org/birth-rate/japan/south-korea
https://georank.org/birth-rate/poland/south-korea
https://georank.org/birth-rate/germany/south-korea

Japan and Poland have been stable, maybe slightly declining, since the early 2000s, and Germany has been stable for 50, all below replacement, whereas South Korea is still going down.

I am no demographic expert, so take this with a grain of salt, but for the moment it doesn’t look like the other countries are going to hit the same problems at the same severity anytime soon whereas South Korea is going to get hit by the full force of their demographic issues within a few short decades.

The growth (or in this case decline) is exponential. 1.08 is bad, 0.80 is terrifying. 1.08 is roughly half of the fertility needed for a stable population (about 2.1 children per woman). This means that per generation your population shrinks by half its size. 0.8 is another 25% lower. At an exponential rate these differences add up fast. The first one gives you (roughly) one child per 8 great grandparents. The other one needs 18. So over 3 generations the population shrinks by another factor of more than a half.