In English, the longest published initialism (in the 1965 edition of Acronyms, Initialisms and Abbreviations Dictionary) is:
ADCOMSUBORDCOMPHIBSPAC is 22 characters and is a United States Navy term that stands for “Administrative Command, Amphibious Forces, Pacific Fleet Subordinate Command.”
In pop culture, the card-game Magic: The Gathering has a playing card called “Our Market Research Shows That Players Like Really Long Card Names So We Made this Card to Have the Absolute Longest Card Name Ever Elemental”, with text on it saying: “Just call it OMRSTPLRLCNSWMTCTHTALCNEE for short.” The initialism is 25 characters long.
The world’s longest acronym, according to the Guinness Book of World Records is NIIOMTPLABOPARMBETZHELBETRABSBOMONIMONKONOTDTEKHSTROMONT (Нииомтплабопармбетжелбетрабсбомонимонконотдтехстромонт). The 56-letter acronym (54 in Cyrillic) is from the Concise Dictionary of Soviet Terminology and means “The laboratory for shuttering, reinforcement, concrete and ferroconcrete operations for composite-monolithic and monolithic constructions of the Department of the Technology of Building-assembly operations of the Scientific Research Institute of the Organization for building mechanization and technical aid of the Academy of Building and Architecture of the Union of Soviet Socialist Republics.”