Complex sentence: If only one of parents held Byelorussian SSR citizenship then citizenship of the child was determined by joint declaration of his parents, without such a declaration child acquired citizenship of Byelorussian SSR by birth, given a right to follow citizenship of another parent upon becoming adult.
Split into simpler sentences: If only one parent held Byelorussian SSR citizenship, then the citizenship of the child was determined by joint declaration of his or her parents. Without such a declaration, the child acquired citizenship of Byelorussian SSR automatically, while still retaining the right to acquire the citizenship of the non-Byelorussian parent upon becoming adult.