If birthrates in Europe and Japan are in decline and if certain areas are being depopulated, then return those areas to nature. Let animals and plants reclaim them. In this day and age when humans have taken over so much of the globe, a nation can always do with more nature.
So, don't see a declining population as a bad thing. Don't feel that towns and villages must be repopulated with immigrants, foreign migrants, or invaders when the native population declines. There is always the wonderful option of letting nature reclaim deserted areas that can be transformed into sanctuaries for birds, mammals, trees, and flowers.
Currently, the Japanese population is 120 million. Let's suppose the population declines to 70 million in 50 yrs. If parts of Japan become emptied of people, then turn those areas into nature preserves. If nature reclaims them, they will remain as parts of Japan. But if foreigners take over, Japan will cease to be Japan.