Complex sentence: He was one of the first Belgian composers to write operas to librettos in Dutch and is known as the composer of the music for De Vlaamse Leeuw, the national anthem of Flanders, for which Hippoliet van Peene wrote the lyrics.
Split into simpler sentences: He was one of the first Belgian composers to write operas to librettos in Dutch. He composed the music for De Vlaamse Leeuw the national anthem of Flanders, and for which Hippoliet van Peene wrote the lyrics.