Boken er på italiensk.

Because boken is the definite singular form of bok.

• bok = book / a book
• boken = the book

In Norwegian, definiteness is often added as an ending on the noun instead of using a separate word like the.

So:

• en bok = a book
• boken = the book

The ending -en marks the noun as definite singular for many common gender nouns in Bokmål.

Here:

• bok = base form
• bok + en → boken = the book

This is a very common pattern in Norwegian:

• en bil → bilen = the car
• en stol → stolen = the chair

So -en is doing the job that English usually does with the.

Er is the present tense of the verb å være = to be.

So:

• jeg er = I am
• du er = you are
• boken er = the book is

Norwegian does not change the present-tense verb depending on the subject, so er stays the same with all persons:

• jeg er
• du er
• han er
• vi er

In Norwegian, when you say that something is in a language, the usual expression is på + language adjective.

So:

• på norsk = in Norwegian
• på engelsk = in English
• på italiensk = in Italian

Even though på often means on in other contexts, here it is just the normal Norwegian way to express language.

So Boken er på italiensk literally looks like The book is on Italian, but the real meaning is The book is in Italian.

Because Norwegian normally does not need the extra word språk here.

Just saying:

• på italiensk

already clearly means in Italian / in the Italian language.

Adding språk would usually sound unnecessary in an ordinary sentence like this.

In Norwegian, names of languages are normally written with a lowercase letter.

So you write:

• norsk
• engelsk
• italiensk
• tysk

This is different from English, where language names are capitalized:

• Norwegian
• English
• Italian
• German

So italiensk is correct here.

Yes. In Bokmål, both boken and boka can be used as the definite singular of bok.

So both of these are possible:

• Boken er på italiensk
• Boka er på italiensk

They mean the same thing: The book is in Italian.

Very roughly:

• boken can sound a bit more traditionally formal or conservative Bokmål
• boka is also very common and natural

Learners will see both.

Here italiensk is functioning like a language term after på.

It is related to the adjective meaning Italian, but in this structure:

• på italiensk

it means in Italian.

You can compare:

• en italiensk bok = an Italian book
• boken er på italiensk = the book is in Italian

So the same word can be used in slightly different ways depending on the sentence.

No, this is very standard Norwegian word order.

• Boken = subject
• er = verb
• på italiensk = complement/adverbial phrase

So the structure is:

Subject + verb + phrase

This is the normal pattern for a simple statement:

• Han er hjemme. = He is at home.
• Filmen er på fransk. = The movie is in French.
• Boken er på italiensk. = The book is in Italian.

A simple approximate pronunciation for an English speaker is:

BOO-ken er paw i-tah-LYENSK

A few helpful notes:

• bok has a long oo-like vowel
• boken is roughly BOO-ken
• på has a long vowel, somewhat like paw
• italiensk has the stress near the end: i-ta-li-ENSK

Pronunciation varies somewhat by dialect, but that approximation is good enough to get started.