Complex sentence: Most of Nielsen's young life was spent in Denmark, until she moved to Germany, where she studied literature and eventually received the highest possible certificate or "abitur" in 1937 from a private school located in Flensburg.
Split into simpler sentences: Most of Nielsen's young life was spent in Denmark, until she moved to Germany. While in Germany Nielsen devoted much of her time to studying literature and eventually received the highest possible certificate or "abitur" in 1937 from a private institution located in Flensburg.