I want to say the book is in the English language not Arabic or French etc. Can I omit 'in' and say 'The book is English.'?
No. I have on my bookshelves a book written in England by an English author in Japanese. The book is English. The book is in Japanese.
"English" is an adjective that denotes nothing about the language used. "in English" tells you that the language used is English but the book might have been written in China by a Dutchman.