Se mi estus aŭtorino, mi ĝojus, ke legantoj volas traduki mian libron en aliajn lingvojn.

The ending -us is the Esperanto conditional mood. It is used for things that are hypothetical, imagined, or contrary to fact.

So:

• Se mi estus aŭtorino = If I were an author
• mi ĝojus = I would be glad

In this sentence, both parts are hypothetical. A more factual version would be:

• Se mi estas aŭtorino, mi ĝojas... = If I am an author, I am glad...

That is just a difference between the two languages.

In Esperanto, when the condition is unreal or hypothetical, it is very normal to use the conditional in the se-clause as well as in the main clause:

• Se mi estus..., mi ĝojus...

English handles this differently, using were in the if-clause and would in the main clause. Esperanto is more symmetrical here.

The suffix -in- marks the feminine form.

• aŭtoro = author
• aŭtorino = female author

So this sentence suggests that the speaker is female, or is imagining herself as a female author.

Because it is not a direct object. It is a predicate noun after esti.

In Esperanto, after esti, the noun or adjective describing the subject normally stays in the basic form:

• Mi estas instruisto.
• Ŝi estas feliĉa.
• Mi estus aŭtorino.

So aŭtorino describes mi; it is not something being acted on.

Esperanto usually separates clauses clearly with commas.

Here you have:

• Se mi estus aŭtorino, ...
  The introductory if-clause is followed by a comma.

• mi ĝojus, ke legantoj volas...
  The ke-clause is also set off with a comma.

This punctuation is very normal in Esperanto and often a bit more regular than in English.

Ke means that and introduces a content clause.

So:

• mi ĝojus, ke... = I would be glad that...

The clause after ke explains what the speaker would be glad about:

• ke legantoj volas traduki mian libron...

Because the wanting is presented as a real fact within the imagined situation.

The sentence means something like:

• If I were an author, I would be glad that readers want to translate my book...

The whole situation is hypothetical because of Se mi estus..., but inside that imagined situation, the readers’ desire is stated as an actual thing: they want.

Also, Esperanto does not automatically shift tenses the way English often does. So the tense in the ke-clause depends on the meaning, not on the tense or mood of the main clause.

If you said volus, that would mean would want and would make the readers’ wanting hypothetical too.

Because after voli (to want), Esperanto normally uses an infinitive for the action wanted.

• voli fari = to want to do
• volas traduki = want to translate

So:

• legantoj volas traduki mian libron = readers want to translate my book

Because mian libron is the direct object of traduki.

In Esperanto, the direct object gets -n:

• libro = a book
• libron = a book, as direct object

And words that agree with the noun also change:

• mia libro
• mian libron

So both words show the accusative:

• mian
  • libron

Usually they do not. But Esperanto can use -n after a preposition to show direction or movement toward a result/place.

Compare:

• en aliaj lingvoj = in other languages
• en aliajn lingvojn = into other languages

With traduki, the idea is changing the text into another language, so en + accusative is very natural.

And again, both words agree:

• aliaj lingvoj
• aliajn lingvojn

Because legantoj is being used in a general or indefinite sense: readers.

So:

• legantoj = readers / some readers / readers in general

If you said la legantoj, it would suggest specific readers already known from context:

• the readers

Esperanto uses la only when the noun is definite.

It is indefinite here.

• aliajn lingvojn = other languages
• la aliajn lingvojn would mean the other languages, a specific set already identified

Since the sentence just means that readers want to translate the book into other languages generally, no article is needed.

A few useful points:

• ĝ sounds like the j in judge
• aŭ is a diphthong, roughly like ow in cow
• oj sounds like oy
• u sounds like oo
• stress is always on the second-to-last syllable

Examples:

• aŭtorino ≈ ow-to-REE-no
• ĝojus ≈ JOY-oos
• aliajn ≈ ah-lee-EYE-n in rough English approximation, with the stress on li because Esperanto stress is penultimate: a-LI-ajn

The exact sound system is more regular than English, so once you know the letters, pronunciation becomes very predictable.