Nowadays there are approximately 18 extant Slavic languages and 400 million speakers of those. Slavic communities are quite fragmented and loosely connected linguistically. Usually, in order to communicate with other Slavic people they use English as a lingua franca. But since Slavic languages are closely related lexically and grammatically and are comparatively easy to learn when another Slavic language is already known, there have been numerous attempts to construct a language that could act as a common language for slavophones instead of English.
Old Church Slavonic probably was the first literary Slavic language, it was spoken in the areas around Thessaloníki and it became widely spoken among many slavic nations. Some scholars believe that Old Church Slavonic was not exactly similar to any Slavic language of that time but was constructed. In any case, Cyril and Methodius and their student Clement of Ohrid created one or more writing systems which serve as a base for many contemporary Slavic languages — Glagolitic or Cyrillic.
In Siberia in 1666, the Croat Juraj Križanić wrote Grammatično Iskazanije ob russkom jeziku (Грамматично исказание об русском езику - Grammatical book of the Russian Language). In this work he described in fact not the Russian language but a Common Slavonic language based on different Slavic languages, mostly on Russian, Serbian and Croatian.
Sample (Romanized, original in Cyrillic): "Iazika sowerszenost iest samo potrebno orudie k mudrosti, i iedwa ne stanowito iee zname. Czim kiu narod imaet izradney iazik, tim prigodnee i witwornee razprawlyaet remestwa i wsakije umitelyi i promisli. Obilie besedi i legota izgowora mnogo pomagaet na mudrich sowetow izobretenie i na wsakich mirnich i ratnich del leznee obwerszenie."
Universalis Lingua Slavica - Universal Slavic language, also known as Vseslovanski jazyk ("All-Slavic language"), is an early example of a zonal constructed language for Slavs. It was created and published by the Slovak Ján Herkeľ in his works Elementa universalis linguae Slavicae and Zaklady vseslovanskeho jazyka in 1826.
Sample: "Za starego vieku byla jedna kralica, koja mala tri prelepije dievice: milicu, krasicu a mudricu; vse tri byle bogate, okrem bogatstva milica byla pokorna, krasica uctiva a mudrica umena." (In olden times there was a queen who had three very beautiful girls: Kindness, Beauty, and Wisdom; all three were rich, in addition to being rich Kindness was humble, Beauty was polite, and Wisdom was wise)
Slava-Esperanto aka Slovina or Slavina (which may be translated as Slavic Esperanto) created by Josef Konečný in 1912 in Prague.
Sample: "Hej, Slované, naši lepo slovanó rěč máme, dokud naše věrne serce pro náš národ dame."
Neposlava (Непослава) was created by Vsevolod Evgrafovich Cheshikhin (Всеволод Евграфович Чешихин) in 1915 in Nizhny Novgorod, Russia. In 1913 he created a system to construct zonal languages based on Esperanto affixes which are used with national roots and called it Nepo. Then in 1915 he created a nepo-language based on the Slavic lexicon - Neposlava ("Slavic Nepo"). He also used this system to construct other "new Esperantoes" based on Latin-Romance and Germanic languages.
Mežduslavjanski jezik (Inter-Slavic language). Work on this language was carried out by a group of Czechoslovak linguists in 1954-1958. The head of the project was the Czech Jiří Karen aka Ladislav Podmele (1920-2000). The language used grammatical and lexical features of Slavic languages, primarily Russian and Czech, and may be viewed as a naturalistic planned language.
Sample: "V meždunarodnich jezikach dlužno stvorit taki cennosti, ktori budu znacit veliki prinos v oblasti nauk, izučenija jezikov, techniki, umenij itd."
After the fall of the USSR, the role of the Russian language as a lingua franca in Eastern Europe and the Balkans diminished. In light of this development, some slavophones have suggested that a new contemporary pan-Slavic language be developed.
A constructed language created in 1999 by Mark Hučko was made as a Slavic variant of Esperanto. Main purposes were to make as simple as possible: for non-Slavic person to learn (no inflection or conjugation; the grammar was based on one of Esperanto) and to write (easy writing system with no diacritics).
But eventually Slovio didn't gain much popularity. A high degree of simplification, characteristic for most international auxiliary languages, makes it easier to learn for non-Slavs; but widens the distance with the natural Slavic languages and gives the language an overly synthetic character, which by many is considered a disadvantage. Also, the predominance of Russian-based words made it not that intelligible for speakers of other Slavic languages.
To fix the problems of Slovio, work on the Slovianski language began in 2006. After gaining popularity a handful of different projects (Slovianski, Novosloviensky jezyk et al.) have merged under the name Medžuslovjanski jezyk (Меджусловјански језык, The Interslavic language). The main purpose of the language was to make it understandable without prior learning, which was achieved using a special voting system for choosing words for the lexicon. Medžuslovjanski jezyk has also gained some attention from the media. As of 2013 approximately 1000 people were able to speak this language.
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