En la terminal, esperamos tranquilas hasta que anuncien el embarque.

Because tranquilas is describing the people, not the manner of the action.

Here, esperamos tranquilas means something like we wait calm / we wait calmly, but Spanish often uses an adjective with verbs like this to describe the subject’s state while doing the action.

So:

• tranquilas = calm, relaxed, feminine plural
• tranquilamente = calmly, in a calm way

In this sentence, Spanish prefers tranquilas because it focuses on how we are while waiting.

It agrees with the implied subject we.

The verb esperamos means we wait / we are waiting, and tranquilas tells you that this we is a group of females.

So the implied subject is basically nosotras.

Compare:

• esperamos tranquilos = we are waiting calmly, masculine or mixed group
• esperamos tranquilas = we are waiting calmly, all female group

Because Spanish often leaves subject pronouns out when they are clear from the verb form.

Esperamos already tells you the subject is we.
And tranquilas adds the extra clue that it is we women / we girls.

So nosotras is unnecessary unless the speaker wants emphasis or contrast.

It can be either, depending on context.

The form esperamos is the same for:

• present: we wait / we are waiting
• preterite: we waited

In this sentence, it is understood as present because of the context:

• En la terminal sets the scene
• hasta que anuncien el embarque refers to a future event from that moment

So here it means we wait / we are waiting.

Because it comes after hasta que and refers to something that has not happened yet.

A very common rule is:

• hasta que + subjunctive when the action is still pending or future
• hasta que + indicative when talking about a habitual action or something already viewed as factual in context

Here, the boarding announcement has not happened yet, so Spanish uses the present subjunctive:

• hasta que anuncien el embarque = until they announce boarding

It does not have to refer to specific people.

Spanish often uses third person plural with no subject to mean people, they, or an unspecified agent. In English, we might say:

• until they announce boarding
• until boarding is announced

So anuncien does not necessarily mean a known group. It just means someone at the airport will make the announcement.

Here, el embarque is a noun meaning boarding.

In airport language, it refers to passengers being called to get on the plane.

So:

• embarcar = to board, to embark
• el embarque = boarding

This is very common in travel contexts, especially at airports.

Yes, by itself it could be.

Embarque can also be:

• a subjunctive form of embarcar
• a formal command form

But in this sentence, it is clearly a noun because it is preceded by el:

• el embarque = the boarding

That article makes the meaning clear.

Because en expresses location: in / at the terminal.

Use en when saying where someone is:

• en la terminal
• en el aeropuerto
• en la puerta de embarque

If you were talking about movement toward the terminal, you would use something different, such as a in another structure.

Not strictly necessary, but it is natural.

En la terminal is an introductory location phrase, and Spanish often allows a comma after that kind of phrase, especially if the writer wants a slight pause or to frame the setting.

So both are possible:

• En la terminal esperamos tranquilas hasta que anuncien el embarque.
• En la terminal, esperamos tranquilas hasta que anuncien el embarque.

The version with the comma sounds a little more segmented or deliberate.

Not in this exact meaning.

If the idea is we are waiting now until that future announcement happens, Spanish wants the subjunctive:

• hasta que anuncien el embarque

Using the indicative, anuncian, would usually suggest a habitual or repeated situation, not a single future pending event.

For example:

• Siempre esperamos hasta que anuncian el embarque.
  = We always wait until they announce boarding.

But in your sentence, the event is still to come, so anuncien is the correct choice.

Esperamos tranquilas is the most neutral order.

The adjective comes after the verb and describes the subject’s state while doing the action. Putting tranquilas first is possible, but it would sound more marked or emphatic.

So:

• esperamos tranquilas = neutral, natural
• tranquilas esperamos = more literary or emphatic, depending on context

The sentence as given uses the normal, everyday order.