Esperanto, the artificial language invented by Dr. L,L, Zamenhof, a Polish philologist, in 1887 — in the hope that it would serve as the universal language — was especially popular among fans in the 1930s and '40s. Its words are based on roots common to many European languages; its spelling is phonetic; its grammar is highly simplified and systematized.

A number of fans have been Esperanto enthusiasts, notably Forry Ackerman and Myrtle R. Douglas, who were among the fans who used Esperanto to coin nicknames (Fojac; Morojo). In addition, some Esperanto expressions crept into Fanspeak, such as "ktp" (from "kaj tiel plu," Esperanto for "and so forth") which is used liberally throughout Fancyclopedia II.

Ed Baker (aka Ejobo in Esperanto) was such a zealous wearer of the Green Star (the Esperanto symbol) that as a member of The Cult he published Fantasy Rotator 116 (1962) entirely in Esperanto. (The Cult immediately found an excuse to drop him and several more to keep him out, but that's another story.)

There was also Dawnish, a language created by Don Wollheim during a period when he was feuding with Ackerman. Dawnish, by definition, was universal, in that no one understood it.

from Fancyclopedia 1 ca. 1944
An artificial language invented for international auxiliary use. The roots for its words come from European language, the root being chosen in each case which occurs in the greatest number of languages; most of them are therefore Latin or Germanic. The spelling is nearly phonetic, and the grammar highly simplified; in addition, there are a number of prefixes and suffixes to extend the vocabulary. Esperi means to hope, espere means hopefully, espero means a hope, and esperanto means one who is hoping; Esperanto was the pename used by the originator. Invented late in the 19th Century, the language became the leading contender for general recognition as an auxiliary language, is taught in many schools, and has thousand of advocates scattered over the world, among whom are Ackerman and Morojo, who have made some converts among fans.