Complex sentence: In French speaking regions, French toast is referred to as "pain perdu", meaning "lost bread", so called because it is a way to reclaim stale or otherwise "lost" bread.
Split into simpler sentences: In French speaking regions, French toast is referred to as "pain perdu", meaning "lost bread" in French. It is referred to as "lost bread" because it is a way to reclaim stale or otherwise "lost" bread.