These are some of the supposed longest words in different European languages:

Irish - “rianghrafadóireachta” - photography

French - “anticonstitutionnellement” - unconstitutionally

Croatian - “Prijestolonasljednikovica” - wife of an heir to the throne

Greek - “ηλεκτροεγκεφαλογραφήματος” - of an electroencephalogram

Latvian - “Pretpulksteņrādītājvirziens” - counter-clockwise

English - “Antidisestablishmentarianism” - against the disestablishment of the Church of England

Swedish - “Realisationsvinstbeskattning” - capital gains tax

Czech - “Nejneobhospodařovávatelnějšímu” - to the least cultivable ones

Polish - “Konstantynopolitańczykowianeczka” the daughter of a man from Constantinople

Norwegian - “Menneskerettighetsorganisasjonene” - the human rights organisations 

Ukranian - “Нікотинамідаденіндинуклеотидфосфат” - nicotinamide adenine dinucleotide phosphate

Serbian - “Семпаравиливичинаверсаламилитипиковски” - (this is actually the last name of a family from Yugoslavia)

Agglutinative languages. Things get even weirder here:

Estonian - “Sünnipäevanädalalõpupeopärastlõunaväsimus” - the tiredness one feels on the afternoon of the weekend birthday party

Dutch - “Hottentottententententoonstellingsterrein” - exhibition ground for Hottentot huts

Hungarian - “Megszentségteleníthetetlenségeskedéseitekért” - for your continued behaviour pretending to be indesecratable

Finnish - “Lentokonesuihkuturbiinimoottoriapumekaanikkoaliupseerioppilas” - (something to do with the Finnish Air Force. Hard to translate but impressively long)

Icelandic - “Vaðlaheiðarvegavinnuverkfærageymsluskúraútidyralyklakippuhringur” - key ring of the key chain of the outer door to the storage tool shed of the road workers on the Vaðlaheiði plateau (Icelandic isn’t even really an agglutinative language which makes this even more impressive)

Turkish - “Muvaffakiyetsizleştiricileştiriveremeyebileceklerimizdenmişsinizcesine” - as though you are from those we may not be able to easily make a number of unsuccessful ones 

And then the longest word is, of course, German. It’s 79 letters long and almost impossible to use in context: 

German - “Donaudampfschiffahrtselektrizitätenhauptbetriebswerk-bauunterbeamten­gesellschaft” - Association for subordinate officials of the head office management of the Danube steamboat electrical services. 

If you know any more impressively long words that I missed, please let me know so I can add them!