I am a native speaker of Russian and I don't only hear long words, I use them: частнопредпринимательский - 25 letters; человеконенавистничество and высокопревосходительство - 24 letters; одиннадцатиклассница and делопроизводительница - 23 letters; the poor grade at school is called неудовлетворительно - 19 letters.
I’ve heard some Filipino teens say “supercalifragilisticexpialidocious”.
The longest word I've ever heard is the English word "smiles". There is a mile between the first s and the second s.
supercalifragilisticexpialidocious is actually from the muscal Mary Poppins and is not a real word
Longer is the name of a Welsh village:
a railway station on the island of Anglesey in Wales, is the longest place name in the Welsh language. 51 letters long in the Welsh alphabet, the name can be translated as "St Mary's church in the hollow of the white hazel near to the rapid whirlpool and the church of St Tysilio of the red cave". However, it was artificially contrived in the 1860s as a publicity stunt, to give the station the longest name of any railway station in the United Kingdom.
The full name of capital city of my country "Krungthep mahanakorn amorn ratanakosin mahintara yutthaya mahadilok phop nopparat rajathani burirom udomrajaniwes mahasatharn amorn phimarn avatarn sathit sakkatattiya visanukam prasit".
And this is the meaning "The great city of angels, the supreme unconqueralble land of the great immortal divinity (Indra), the royal capital of nine noble gems, the pleasant city with plenty of grand royal palaces, and divine paradises for the reincarnated deity (Vishnu), given by Indra and created by the god of crafting (Visnukarma)".
Now it's shorted to "Krungthep mahanakorn" (The great city of angels) and called shortly "Krungthep" (The city of angels).