¿En qué países de Europa tendrán
mayores problemas debido a la disminución de población?

The vast majority of European countries will have problems because of population decline, with the ones in the eastern half of the continent being more affected.

Explanation:

As a whole, Europe is a continent that is having big problems with population decline. Just three countries in the continent experience slight population growth, while all others either stagnate or experience a decline. Now when it comes to birthrates and death rates, the differences are minuscule across the continent, but migration is something that makes the difference better.

While both the western and eastern half have low birthrates, the western, richer side doesn't really lose population but has a rough stagnation. This is due to the fact that it is an attractive destination for migrants, including migrants from the poorer eastern half of Europe, so the numbers are relatively static. The eastern half of Europe though, not just has low birthrates, but it also losses a lot of its population because it migrates toward the western half in search of a better life. Also, it is not very attractive for migrants from other regions, so it experiences a rapid population decline. The two most extreme cases in the last three decades are Macedonia and Bulgaria, each losing roughly between 20 and 25% of their populations.