The Czechoslovak language (Czech: jazyk československý) was a political sociolinguistic concept used in Czechoslovakia in 1920–1938 for the definition of the state language of the country which proclaimed its independence as the republic of two nations, Czechs and Slovaks.
On 29 February 1920 the National Assembly adopted the Czechoslovak Constitution and, on the same day, a set of constitutional laws. The Language Act (Jazykový zákon) 122/1920 Sb. z. a n., on the grounds of § 129 of the Constitutional Charter (Czech Ústavní listina Československé republiky) has set the principles of the language regulations, where § 1 ruled that the Czechoslovak language "jazyk československý jest státním, oficielním jazykem republiky" ('is the state, or official language of the republic').
In practice, in the international documents this role was played by the Czech language. Meanwhile, the Constitution of 1920 and its derivative acts allowed the usage of minority languages provided they were spoken by not less than 20% of the local population of certain areas.
Officially the 1920 constitution was superseded on 9 May 1948 with the Ninth-of-May Constitution where the concept of the official language was omitted.
The "Czech-Slav Society" created in 1829 by students of the Lutheran Lyceum in Bratislava, also called the "Society for the Czechoslovak language and literature", became an important entity in the Slovak national movement.
In 1836, Ľudovít Štúr, the leader of the Slovak national revival in the 19th century wrote a letter to the important Czech historian František Palacký. Stating that the Czech language used by the Protestants in Upper Hungary had become incomprehensible for the ordinary Slovaks, Štúr proposed to create a unified Czechoslovak language, provided that the Czechs would be willing to use some Slovak words – just like Slovaks would officially accept some Czech words.
However in the first half of the 20th century the radical concept of "Czechoslovakism" set forward the Czech language as the literary norm, while the Slovak language was considered to be a local dialect, as was the Moravian language. The concept of Czechoslovakism was necessary in order to justify the establishment of Czechoslovakia to the world, because otherwise the statistical majority of the Czechs as compared to Germans would be rather weak.