Complex sentence: In 1779 he was invited to sing for the Russian Crown Prince (later to become Tsar Paul I) in Stuttgart, where he was discovered by one of the Elector's "talent scouts" and immediately engaged to work in Munich.
Split into simpler sentences: In 1779 he was invited to sing for the Russian Crown Prince (who became Tsar Paul I) in Stuttgart. During his time in Germany he was discovered by one of Paul's "talent scouts" and immediately engaged to work in Munich.