Other comments had it so I didn't think it was necessary. Immigration can prop up a low birthrate but that can't last forever. Need to actually have a culture that supports procreation. And Japan doesn't really have that. Their work culture is directly responsible for it. I don't think that's something easily fixed. Financial incentives could help, but unless it's pretty hefty it probably wouldn't be enough.
But we don't have an 80hr work week as the norm and we can piss off straight after work without feeling the need to have a beer with our colleagues or bosses.
It did for a few hundred years before they became a vassal state of the US .. and wouldn't you know it the US is also in a birth rate crisis.
Isolationist culture is fine, you just can't mix it with the crushing reality of capitalism and it's negative effects on the ability of people to raise families.