Complex sentence: In the years before the First World War, the movement had achieved "de jure" legal equality for Dutch as an official national language, but the issue was still contentious at the outbreak of war in August 1914.
Split into simpler sentences: In the years before World War I, the movement achieved "de jure" legal equality for Dutch as an official national language. However, the issue remained contentious at the outbreak of war in August 1914.