This is the 9953rd most frequent French word.
francais
French; relating to France or the French language.
Here, "français" is used as a noun to refer to the French language, indicating a skill or ability in speaking it.
Je parle français couramment.
I speak French fluently.
In this instance, "français" functions as a noun describing the language in which the book is written.
Ce livre est écrit en français.
This book is written in French.
Here, "Français" is used as a plural noun to represent the people of France, indicating nationality or identity.