* No official census: There's no official census in France that tracks language proficiency.
* French is the official language: French is the official language of France, so virtually everyone in Paris understands and speaks it to some degree.
* Immigrant communities: Paris has a large immigrant population, some of whom may speak other languages as their primary tongue. While they might be fluent in French, it might not be their first language.
However, it's safe to say that the vast majority of Parisians speak French, and it's the primary language for most.