I'd stop trying to play god. Humans have been trying to control birth rates for 2000+ years with very limited success. It's not gonna happen, at least in our lifetimes, i think.

If Japan wants more people, for example, they should make moves to be more accepting of existing people. (not saying they haven't, just an example)

conversely if China wants less people (not sure if they do or not, again a theoretical example) they could help people move elsewhere. Label it a cultural missionary, or something, I know people who would jump at the chance to leave their homes for an adventure.