Very interesting display, thanks.
Poland has been decreasing for years. Why? A few reasons possibly. For one I think the population of Poland itself has been declining due to migration. This doesn't explain the extent of JW decline however. Initially Poland was one of JWs' greatest successes in Eastern Europe in the 1990s. So as with Japan, Poland's decline might to some extent be in proportion to its previous meteoric rise. Poland also has a strong Catholic national identity to fall back on of course. There is a strong "anti-sect" discourse in Eastern Europe, especially relating to JW refusal of blood. East Europeans find this disregard for human life particularly offensive. Plus nationalism has been on the increase in Poland as the rest of Europe. As has been mentioned, Polish apostates seem to be active and effective as well.
Another statistic I find intriguing is the increase in Ukrainian congregations at a time when their publisher numbers are decreasing. If it wasn't for the increase in Ukrainian congregations then the Europe wide figure would be over 100 congregations lost.
Netherlands and Denmark continue long term decline.
Baptisms are down in the majority of countries, even as the WT has stressed getting baptised young,
Greece lost 5% of its congregations in a single year, which is higher as a proportion than the United States.
An interesting statistic I thought might be worth calculating is baptisms per publisher. This would probably be a good indication of future decline because you need a certain number of baptisms each year to cover deaths and losses. Worldwide the figure is over 3%, but for declining countries it is much lower. Poland has just over 1% which I think is the worst in Europe and Japan has just under 1% which I think is the worst in the world. As long as baptisms remain so low these countries will continue long term decline. When JWs were growing strongly in the 1980s and 1990s they were baptising 7 or 8% of their publisher numbers per year.