"This book is in Persian, not in Turkish."
Translation:Bu kitap Farsça, Türkçe değil.
Really coming from a historical perspective, it is an adverb. It would mean "like a Persian." Language names in general have a lot of blurred lines in Turkish. They normally do not have case endings on them (or rather, they have them in fewer cases than nouns) and behave like adjectives, nouns, and adverbs.
Imagine it here like it means something like "in the style of Persia" :)
I copy my comment from another discussion, it may help: " Names of languages are always nouns but you can see them used like adjectives like in the case "Türkçe Kitap" which means "book in Turkish". This doesn't have the same meaning with "Türkçe Kitabı" which means a "book which teaches Turkish". In the former case you see a noun pretending as an adjective since "Kitap" doesn't have any suffix. But actually even in that case Turkish grammar doesn't take it as an adjective. This is another type of noun phrase which is called "takısız isim tamlaması".
In case of nationalities you need to use them as adjectives.
Example: İngiliz futbolcu
This exactly means the soccer player's nationality.
But check the example: İngiliz atı
Here we need the suffix "-ı" for "at" (horse) since it is not a nationality, it is an origin, a race.