BONN - Chancellor Helmut Kohl's Christian Democrats were set to agree to a plan yesterday which would for the first time concede the principle that German nationality is not only inherited, but can be acquired by being born in the country. The CDU leadership was debating a proposal which would give dual nationality to immigrants children born in Germany. Germany is the only major western industrial nation to maintain the principle, enshrined in an imperial law dating back to 1913, that nationality is essentially passed down through the blood line.