Currently on TMDB this TV show has 4 season each with 26 episodes (everything only in German), as it was created after fernsehserien.de. However according to the official site there are in fact only 2 seasons each with 52 episodes. I'm going to correct this and I'm leaving this comment to let others know why I did such change and what were my sources. I'm also adding french and czech translations and I'm removing original air dates as they refer only to the first german broadcast, not to the original air date).

Other sources proving there are only 2 seasons:
Episode listings on allocine.fr: http://www.allocine.fr/series/ficheserie-4303/saisons/
French wikipedia: https://fr.wikipedia.org/wiki/Le_Petit_Nicolas_(s%C3%A9rie_t%C3%A9l%C3%A9vis%C3%A9e_d%27animation)