Germany – Austria – Luxembourg – Switzerland Donaudampfschiffahrtselektrizitätenhauptbetriebswerkbauunterbeamtengesellschaft
Belgium Donaudampfschiffahrtselektrizitätenhauptbetriebswerkbauunterbeamtengesellschaft With three official languages – Dutch, French and German – the longest word in Belgium is then logically the German one.
Italy Precipitevolissimevolmente
Czechia Nejneobhospodařovávatelnějšímu
Slovakia Znajneprekryštalizovávateľnejšievajúcimi
Poland Konstantynopolitańczykowianeczka
Lithuania Nebeprisikiškiakopūsteliaudavusiuose
Latvia Pretpulksteņrādītājvirziens
Estonia Sünnipäevanädalalõpupeopärastlõunaväsimus
Belarus Электрапаветраразмеркавальнік
Ukraine Нікотинамідаденіндинуклеотидфосфат
Romania – Moldova Pneumonoultramicroscopicsilicovolcaniconioză
Okay, I got this list from here and I was kind of done with removing the text in between and giving them bulletpoints. Not going to place them in alphabetic order myself ;)