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

Almost all of the European countries will have major problems because of population decline.

Explanation:

Europe, as a continent, has the biggest problems when it comes to population aging and population decline. There are only three countries that experience population growth without using large-scale migration to fill in the gaps, those three being Ireland, Albania, and Kosovo. All of the others have negative population growth or stagnate and don't have a decline because they allow large numbers of migrants to work and live there.

Because the countries in the western half of the continent are richer and have many pull factors for migrants, it is likely that they will manage to balance the population around some number for at least some time, so they shouldn't really experience that much of a problem with population loss, though they will have a lot of social and political ones. On the other hand, the countries in the eastern half of Europe will have huge problems because not just that their birthrates are low, but they lose a lot of population through migration, and they don't really have any particular pull factors for migrants from other countries to be attracted to them. The situation is even now very bad in some of these countries, with Bulgaria and Macedonia being the most affected, both having lost between 20 and 25% of their population in the last three decades.