I am more on the beginner side and try to read "Zazie dans le métro" by Raymond Queneau.
There is a phrase I can not fully understand:
– Oui, dit Zazie, je veux être institutrice.
– Ce n'est pas un mauvais métier, dit doucement Marceline. Y a la retraite.
Elle ajouta ça automatiquement parce qu'elle connaissait bien la langue française.
The last sentence doesn't make much sense to me. How is it related to French language? I could not find an English translation online, but my Russian translation says something like "she added that automatically, she knew well not just French language, but also French mentality". So I assume it's kind of wordplay around word "langue", but I can not find anything in dictionaries.