"Technically, Slovakia is not the successor state of Czechoslovakia, Czechia is. If a person was born in Czechoslovakia, and only speaks the official language of the successor state of Czechoslovakia, then that person should probably be a Czech citizen, not Slovak."
> Most varieties of Czech and Slovak are mutually intelligible, forming a dialect continuum (spanning the intermediate Moravian dialects) rather than being two clearly distinct languages.
https://en.wikipedia.org/wiki/Czech%E2%80%93Slovak_languages
Technically, Estonia is not the successor state of the USSR, Russia is. If a person was born in the USSR, and only speaks the official language of the successor state of the USSR, then that person should probably be a Russian citizen, not Estonian.