In the 1630s, the priest Wojciech Dembołęcki wrote in The Origin of the Omnipotent World State that in paradise, God spoke to Adam and Eve in Polish. Similar claims, which today seem a manifestation of national megalomania, used to appear in the chronicles of various nations speaking various languages.
Although science has made enormous progress since Dembołęcki’s time, we still don't know what language was spoken by the first people. We don’t even know if all languages have a common ancestor. This concept, which refers to the biblical Tower of Babel, cannot be proved and is rather questionable in view of the enormous diversity of the over five thousand languages spoken in the world. That being said, there are many languages which are geographically distant and too different to make any communication possible but show surprising traces of common origin in their grammatical structure and vocabulary. We can therefore deduce common origins of languages within a dozen or more language families.
The best-known language family, spreading over the largest area and with the largest number of speakers, is the Indo-European family, named after the two geographical extremes of the area it covers.
Finding traces of a shared history is not easy, as similarities between languages may be a result of later contacts between nations. In the area of Indo-European languages, there are also numerous “islands” of genetically different languages that were once brought by migrating peoples. The non-Indo-European languages which are the closest to us include Hungarian, Finnish, Estonian and Turkish.
In the modern languages spoken by our eastern and southern neighbours, it is much easier to see evidence of the kinship of Slavic languages than traces of the Pre-Indo-European family. These modern languages descend from a common Proto-Slavic language which belonged to the family of Indo-European languages.
While we do not know for sure where the Proto-Slavs lived, we do know that three subgroups of Slavic languages emerged as a result of the migration of peoples: South Slavic – in the area between Romania, Hungary, Austria and the Balkans; East Slavic – east of Poland; and finally, West Slavic, which includes, apart from Polish, the languages that resemble it the most: Slovak, Czech, Lower and Upper Sorbian.
Nowadays, Kashubian speech, which was treated as a dialect just a few dozen years ago, is also widely recognized as a separate Slavic language. The difference between a dialect and a language depends more on politics than on the structure of the language.
Source: encyklopedia.pwn.pl/materialy-dodatkowe; jezyki.studentnews.pl