All of these languages were very similar to each other more than 100 years ago but were actively forced to diverge for political reasons during the turbulent 20th century.
Czech/Slovak/Polish
Macedonian/Serbian/Bulgarian
Croatian/Slovenian
Of course outliers like Romanian/Greek/Turkish/Albanian/Hungarian are different by construction. But even they shared extensive vocabulary with the rest due to the free flow of (rich) people between the countries.
The forced divergence for political reasons might be true for the Balkan languages, but not for the Central European ones. The only artificial force in Czech has been the attempt to get rid of the German influence, and judging by the huge similarities in morphology of Czech to German, it wasn’t very successful.
All of these languages were very similar to each other more than 100 years ago but were actively forced to diverge for political reasons during the turbulent 20th century.
Czech/Slovak/Polish
Macedonian/Serbian/Bulgarian
Croatian/Slovenian
Of course outliers like Romanian/Greek/Turkish/Albanian/Hungarian are different by construction. But even they shared extensive vocabulary with the rest due to the free flow of (rich) people between the countries.