The Czech language is one of the West Slavic languages, along with Slovak, Polish, Pomeranian (Kashubian), and Lusatian Sorbian. It is spoken by most people in the Czech Republic and by Czechs all over the world (about 12 million native speakers in total).
Czech is quite close to and mutually intelligible with Slovak and, to a lesser degree, to Polish or to Sorbian in East Germany. As for the relation between Czech and Slovak, speakers of these languages usually understand the other language pretty well both in its written and spoken form and together they constitute kind of language diasystem, though some dialects or heavily accented speech might present difficulties to them (in particular, geographically most distant Eastern Slovak dialects to Czech speakers).
Source - Wikipeadia.com
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