This is the 6356th most frequent Russian word.
французской
The word "французской" in English means "French" (feminine singular or feminine genitive/dative case in Russian).
Here, "французской" is used as an adjective describing the language the book is written in.
Книга написана на французской языке.
The book is written in French language.
Here, "французской" is an adjective describing the type of cuisine the speaker enjoys.
Я люблю французской кухни.
I love French cuisine.
In this case, "французской" describes the fashion being referred to, indicating it's French style.