Complex sentence: He devoted his leisure hours to visiting the city's museums, art galleries and opera houses for which he held a great affection and learned to speak the German language which he gained a reasonable fluency in by the time he was 12.
Split into simpler sentences: He devoted his leisure hours to visiting the city's museums, art galleries and opera houses, which he held in great affection. He gained a reasonable fluency in German by the time he was 12.