The longest officially used German word was:

Rindfleischetikettierungsüberwachungsaufgabenübertragungsgesetz

(63 letters)

Translation:

“Beef labeling monitoring delegation law”

This was the name of a law in the German state of Mecklenburg-Vorpommern. It referred to the delegation of tasks related to monitoring beef labeling, especially relevant during the BSE (mad cow disease) crisis. The law was abolished in 2013.

But German allows theoretically endless compound words. A classic (but not officially used) example is:

Donaudampfschifffahrtselektrizitätenhauptbetriebswerkbauunterbeamtengesellschaft

(80 letters)

Translation:

“Association of subordinate officials of the main building for electrical services of the Danube steamship company”

This is a joke word created to illustrate how German compounds work. It’s not used in real life, but it’s popular in linguistic discussions and trivia.

Theoretically you can make infinitely long words in German.

Supercalifragilistichespiralidoso

In Estonian, we traditionally consider the word kuulilennuteetunneliluuk - the hatch of the tunnel for the bullet’s flight path, to be the longest, but as with German, arbitrarily large words are possible.

Like with German, we can make arbitrarily long compound words in Dutch (for example, “kindercarnavalsoptochtvoorbereidingswerkzaamhedencomitéleden”), but if we limit it to words that are in the dictionary, “arbeidsongeschiktheidsverzekering“ (disability insurance) is one of the longest words that’s also commonly used.

Not the longest ones but the first that came to mind:

In French, one of the longest words is “anticonstitutionnellement”, which means unconstitutionally. It’s long but not something you’d use in daily conversation.

For Japanese, it’s trickier because the language doesn’t really form long standalone words like German does. But if we count kanji compounds, something like “超電磁砲” (ちょうでんじほう, chōdenjihō), meaning railgun, is a cool one. Of course, technical or bureaucratic terms can get way longer, but they’re more like phrases than words.

Anticonstitutionnellement

In a manner that is against the constitution.

Heksakosioiheksekontaheksafobia

phobia to numbers 666

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We’ll have lots of English speakers here given the language the question was asked in, so I’ll do Gàidhlig (Scottish Gaelic) instead: dìochuimhneachadh, at 17 letters. It means “forgetting”, and it is pronounced /ˈd̥ʲĩə̃xənəxəɣ/. No, I can’t say it smoothly.

Gàidhlig isn’t one of those languages that can compound words like Finnish or German, this one is just a consequence of a few different things. Firstly, the language’s spelling rules result in a lot of letters that do impart information but aren’t directly pronounced. Consonants have two forms depending on which of two sets of vowels they are next to, so any consonant or consonant cluster must always have vowels from the same set on either side. For example, the “i” in the “imhne” bit in the middle is basically only there to match the “e” at the end, since u and e aren’t in the same set of vowels and we need to know which version of the consonants between them to use. Every h is a modifier on the consonant preceding it as well. Second, the root of it is “un-remember”, so it’s already a shorter word with a prefix. Third, we’re using the verbal noun version, so it’s “the act of forgetting” rather than present-tense as in “currently forgetting something”

There are probably longer words in the language, but I don’t know it very well yet and this was the longest one I could find on a word list. I think there’s actually a version of dìochuimhnich that includes a suffix marking it as being a conditional first person plural doing the forgetting, so “we would forget”, but I don’t understand how that part of the language works. If I was to say that at the moment, I would use two words to do it, so I don’t feel like I can give it as an answer here

Lithuanian: “Nebeprisivaizdotinklaraštininkaujantiesiems”

Meaning: To those who are no longer satiated by the amount of vlogs they create

In English, I understand it to be pneumonoultramicropicsilicovolcaniconiosis, although I may have misspelled it from memory. It’s a medical condition resulting from inhaling fine particles of silica.

In Finnish language you can compose compound words freely, so the question is a bit odd. Your imagination is the limit.

In English it’s usually “FUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU–…” (trimmed due to character limit per post)

Meaning varies with context, but it’s most often an assessment of current events.

The actual longest word according to the Swedish academic word list is “Realisationsvinstbeskattning” (Capital gain taxation)

But as with many languages we can add together several words in some ways so according to Guinness world records (yeah, they suck) it’s “nord-väster-sjö-kust-artilleri-flyg-spanings-simulator-anläggnings-materiel-underhålls-uppföljnings-system-diskussions-inläggs-förberedelse-arbeten.” (North western lake coast artillery flight reconnaissance simulator construction material maintenance follow-up system discussion posts preparation work.)

Here are the longest words in Esperanto. I’m not fluent though, still learning.