Mi demandis la vendistinon, ĉu tiu romano estas originala aŭ traduko el alia lingvo.

Because la vendistinon is the direct object of demandis.

• Mi demandis la vendistinon = I asked the saleswoman
• The ending -n marks the accusative, which usually shows the direct object in Esperanto.

So here, the person being asked is marked with -n.

Because Esperanto can use demandi directly with an embedded question.

So:

• Mi demandis la vendistinon, ĉu... = I asked the saleswoman whether...

If you use a noun instead of a full question, then pri is common:

• Mi demandis la vendistinon pri la romano = I asked the saleswoman about the novel

So pri is not needed before ĉu.

Here ĉu introduces an indirect yes/no question, so it means whether or sometimes if.

• Mi demandis ..., ĉu tiu romano estas originala...
  = I asked whether that novel is original...

Compare a direct question:

• Ĉu tiu romano estas originala aŭ traduko el alia lingvo?
  = Is that novel original or a translation from another language?

So ĉu is the normal word for introducing this kind of question in Esperanto.

Because tiu romano is the subject of estas inside the subordinate clause.

In other words:

• tiu romano estas originala = that novel is original

Subjects do not take -n.
You would use -n only if romano were a direct object, for example:

• Mi aĉetis tiun romanon = I bought that novel

That is a very natural question. After esti, Esperanto can use either:

• an adjective: estas originala
• a noun: estas traduko

So grammatically, both work.

The sentence is contrasting two possibilities:

• originala = original
• traduko = a translation

This is understandable, although some learners may expect a more perfectly parallel structure. More symmetrical alternatives would be:

• ĉu tiu romano estas originala verko aŭ traduko el alia lingvo
• ĉu tiu romano estas originala aŭ tradukita

But the given sentence is still clear.

El alia lingvo means from another language.

With translations, Esperanto normally uses el to show the source language:

• traduko el la angla = translation from English
• traduko el la franca = translation from French

Here alia lingvo means some other language, not a specific named one.

Using el is standard because it suggests the text has been translated out of that language.

Because alia lingvo is indefinite here: it means another language, not the other language.

So:

• el alia lingvo = from another language

If a specific language were meant, Esperanto would usually name it:

• el la hispana
• el la germana

Or, in a context where a specific other language had already been identified, the wording would likely be different.

It breaks down like this:

• vend- = sell
• -ist- = person զբաղוקita in an occupation or activity
• -in- = female
• -o = noun ending
• -n = accusative ending

So:

• vendisto = seller, salesperson, shop assistant
• vendistino = female seller / saleswoman
• vendistinon = the accusative form of vendistino

This kind of word-building is very common in Esperanto.

Because Esperanto commonly uses commas to separate subordinate clauses from the main clause.

So in:

• Mi demandis la vendistinon, ĉu...

the comma marks the start of the indirect question. This is standard punctuation in Esperanto.

Because tiu means that, while ĉi tiu means this.

So:

• tiu romano = that novel
• ĉi tiu romano = this novel

The sentence is referring to a particular novel as that novel. If the speaker wanted to say this novel, they would use ĉi tiu romano instead.

Yes. This is a common false friend for English speakers.

In Esperanto:

• romano = novel

It does not specifically mean romance in the English sense. If you want to describe a love-story novel, Esperanto often uses a more specific expression such as amromano.