Complex sentence: Paris Saint-Germain is the only club to have never been relegated from Ligue 1, and one of only two French clubs to have won European titles--the other being arch-rivals Olympique de Marseille.
Split into simpler sentences: Paris Saint-Germain remain the only club to have never been relegated from Ligue 1. The Parisians are also one of only two French clubs to win a European title--the other being arch-rivals Olympique de Marseille.