A editora vai publicar o livro no próximo mês.

Because editora is a feminine noun in Portuguese, so it takes the feminine definite article a.

• a editora = the publisher / the publishing house
• o editor = the editor / publisher (masculine)

In this sentence, editora most naturally means publishing house or publisher as a company.

Here it most likely means publisher or publishing house, not female editor.

That is because of the context:

• publicar o livro = to publish the book

A company publishing a book is a very natural meaning here, so a editora is best understood as the publisher / the publishing house.

In other contexts, editora can also refer to a female editor, but that is less likely in this sentence.

Vai publicar is the very common near future / going to structure:

• vai = goes / is going
• publicar = to publish

Together, vai publicar means is going to publish / will publish.

Portuguese also has a simple future form:

• publicará = will publish

Both are correct, but vai publicar is very common in everyday spoken Portuguese and sounds natural.

Because after ir used for the future, Portuguese uses the infinitive, not another conjugated future form.

So:

• vai publicar = correct
• vai publicará = incorrect

This works like:

• vou comer = I am going to eat
• vamos sair = we are going to leave

The pattern is:

ir (conjugated) + infinitive

Portuguese uses definite articles more often than English does.

So o livro literally means the book, and in many contexts Portuguese prefers that article where English might or might not use it.

Here, o livro suggests a specific book that is already known in the context.

No próximo mês means next month or more literally in the next month / in the coming month.

No is a contraction:

• em + o = no

So:

• no próximo mês = in the next month

This contraction is very common in Portuguese:

• na cidade = in the city (em + a)
• nos livros = in the books (em + os)
• nas escolas = in the schools (em + as)

In Portuguese, adjectives can come before or after the noun, but the position often depends on the word and the expression.

With time expressions like this, próximo usually comes before the noun:

• o próximo mês = the next month
• a próxima semana = next week
• o próximo ano = next year

So no próximo mês is the normal way to say next month here.

Yes. That is correct and means essentially the same thing:

• A editora vai publicar o livro no próximo mês.
• A editora publicará o livro no próximo mês.

The difference is mainly style and register:

• vai publicar = very common, natural, especially in speech
• publicará = slightly more formal or written

This word order is the most neutral and natural one:

A editora vai publicar o livro no próximo mês.

Portuguese does allow some variation, but changing the order can sound more marked or put emphasis on a certain part.

For example:

• No próximo mês, a editora vai publicar o livro.

This is also correct, but now no próximo mês is being highlighted.

A careful European Portuguese pronunciation would be approximately:

A editora vai publicar o livro no próximo mês.
uh ee-dee-TOH-ruh vai poo-blee-KAR oo LEE-vroo noo PRO-see-moo mesh

A few useful notes for Portugal Portuguese:

• Unstressed e and o are often reduced.
• Final -e can sound very weak.
• s at the end of mês sounds like sh in European Portuguese.

So mês sounds close to mesh.

Often yes, because the verb ending can show who the subject is. But in this sentence the subject is a noun phrase, not a pronoun:

• A editora vai publicar o livro...

If you remove a editora, you get:

• Vai publicar o livro no próximo mês.

That could mean He/She/It is going to publish the book next month, depending on context, so it becomes less clear. Keeping a editora makes the subject explicit.

Yes, in this sentence it matches English next month very well.

Portuguese often uses:

• no próximo mês = next month
• na próxima semana = next week
• no próximo ano = next year

In some contexts, Portuguese can also say simply:

• próximo mês

But no próximo mês is very natural and complete here.