"The Czechoslovak language (Czech: jazyk československý, Slovak: Československý jazyk) was a political sociolinguistic concept used in Czechoslovakia in 1920–1938 for the definition of the state language ...
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of the country which proclaimed its independence as the republic of two nations, i.e. ethnic groups, Czechs and Slovaks.
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." - (Wikipedia (en) 08.09.2021)