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.

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.

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.

Heksakosioiheksekontaheksafobia

phobia to numbers 666

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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.

The Chinese language doesn’t quite work that way as it is based almost solely on distinct characters…

I guess you can just keep compounding characters together. Just as a quick example, “[the] People’s Republic of China” is a 7-character word in Chinese with no breaks… it can go much, much longer as necessary, but I’m not sure if that counts, since it’s essentially just three words joined together (“China”, “People”, “Republic”)

Otherwise, the closest thing might be some of the longer Chinese idioms (“Chengyu”), although most Chengyus are only 4 characters long

Learning a language where you need to know how to write thousands of differently squiggles (with almost no rules whatsoever) to even communicate is difficult in its own way though