Complex sentence: The first programming language to be proposed was Plankalkül, which was designed by Konrad Zuse in the 1940s, but not publicly known until 1972 (and not implemented until 1998).
Split into simpler sentences: The first programming language to be proposed was Plankalkül, designed by Konrad Zuse in the 1940s. However, it's existence was not publicly known of until 1972 (and not implemented until 1998).