It's not just Japan, though the media seems fixated on Japan to an unusual extent.
Here are some 2023 TFR estimates, source: CIA.
Japan: 1.39. Poland: 1.41. Spain: 1.29. South Korea: 1.1. Taiwan: 1.09. China: 1.45. Canada: 1.57. Germany: 1.58. UK: 1.63. Cuba: 1.71. USA: 1.85.
Italy: 1.24. This is noteworthy as Italy has been under replacement TFR for nearly 50 years. (Since 1975.)
Hungary: 1.49. Despite best efforts.
Just about every developed country on Earth is presently experiencing sub-replacement TFR, and a handful of large countries are worse-off than Japan, or in comparable positions.
Why all of the articles about Japan, when the situation in Italy is more dire? Go figure.
What is to be done? What's clear is that only radical solutions have any chance at working.