In Czech it's nejneobhospodařovávatelnější. If we count declined forms, it would be nejneobhospodařovávatelnějšími.
Rindfleischetikettierungsüberwachungsaufgabenübertragungsgesetz
(das, 63 letters)
This hyper word references a "beef labeling regulation and delegation of supervision law." This was a 1999 German Word of the Year, and it also won a special award as the longest German word for that year. It refers to a "law for regulating the labeling of beef"—all in one word, which is why it is so long. German also likes abbreviations, and this word has one: ReÜAÜG.
There could even be one out there somewhere that beats this one. This was the winner for 1999.
Ever notice how English hates long words? Even a word like „antidisestablishmentarianism“ is nothing compared to German words, and those in other languages. Antidisestablishmentarianism looks like just an average sized German word to me, that’s not particularly long.
I think the reason all ties into grammatical gender and the lack of it in English. That allows nouns to be separated and placed side-by-side in English. You couldn’t do that in German or probably any language with grammatical gender because you’d have to ask: which gender would or could you assign to a combination of words in separation to one another? It doesn’t make any sense. Hence, you have to combine them and assign one gender, in German at least just assigning the gender of the last word in the combination.
The German example is just 6 words all put together with a „das“ gender because the last word in the combo, „Gesetz“ is a neuter „das.“ But split the words up and you can get differing genders. That’s the problem.
I think that’s the reason for longer words in languages with grammatical gender.