In a related question, @defaultlocale posted this excerpt:
Article 2. Possession of Latvia citizenship Citizens of Latvia are:
- persons who were citizens of Latvia on June 17, 1940 and their descendants who have registered according to the procedures established by law, except persons who have become citizens (subjects) of another state after May 4, 1990;
- persons who have obtained the citizenship of Latvia through naturalization or another manner according to the procedures established by law;
- children found within the territory of Latvia whose parents are not known;
- children with no parents who live in an orphanage or boarding school in Latvia;
- children both of whose parents were citizens of Latvia on the day of birth of such children, regardless of the place of birth of such children.
Law of citizenship, Latvia, 1994
But how would a person living in Estonia or Latvia prove that their family settled in the country over 50 years ago? Both countries were occupied by Nazi Germany in 1941, so presumably many documents were irrevocably lost during the war. Was it based on knowledge of the local language? Based on names and surnames? Or is my assumption incorrect and everyone's family history was available in government records?