Sem a coleira, a cadela pode começar a latir para toda a gente na rua.

Because Sem a coleira is a fronted phrase that sets the condition or situation for the rest of the sentence: without the leash / when she doesn’t have the leash on.

The comma is very natural here because it separates that introductory idea from the main clause:

Sem a coleira, a cadela pode começar a latir...

In English, this is similar to writing:

Without the leash, the dog may start barking...

A comma is not always strictly required after a short opening phrase, but in this sentence it sounds clear and natural.

Portuguese uses definite articles much more often than English.

So where English might say:

Without a leash, the dog...

Portuguese often says:

Sem a coleira, a cadela...

Here, a cadela means the female dog, and a coleira means the leash/collar.

This usually suggests a specific dog and her leash, not dogs in general. Portuguese commonly uses the article even when English would leave it out.

Yes. Both are possible, but they are slightly different in feel.

• sem a coleira = without the leash, probably a specific leash already known from the context
• sem coleira = without a leash / unleashed, more general

So:

• Sem a coleira, a cadela... sounds like we are talking about that dog and her leash
• Sem coleira, a cadela... sounds a bit more general: if she is not wearing a leash

Both are natural, but sem a coleira is more specific.

Cadela specifically means female dog.

Compare:

• cão = male dog, or sometimes dog in a general sense
• cadela = female dog

So the sentence is specifically about a female dog, not just any dog.

A learner may notice that English bitch can mean female dog, but in English that word is often rude or insulting in everyday use. In Portuguese, cadela is the normal neutral word for a female dog.

After começar, Portuguese normally uses a + infinitive to mean to begin/start doing something.

So:

• começar a latir = to start barking
• começar a correr = to start running
• começar a falar = to start speaking

In this sentence:

• pode = can / may
• começar a latir = start barking

So pode começar a latir means may start barking or can start barking.

That a is just part of the normal structure after começar.

Because the meaning is slightly different.

• pode latir = she may bark / she can bark
• pode começar a latir = she may start barking

The version with começar a focuses on the beginning of the action. It suggests that once the dog is without the leash, barking may begin.

So this is not just about the general ability to bark; it is about the barking starting in that situation.

Toda a gente means everyone / everybody.

Literally, it looks like all the people, but grammatically it behaves as a singular expression in Portuguese.

For example:

• Toda a gente sabe. = Everybody knows.

So even though the meaning is plural in English, the grammar in Portuguese is usually singular.

In your sentence, it comes after para, so you do not see verb agreement, but the expression itself is still treated as singular.

Here para marks the target of the barking: at everyone.

So:

• latir para toda a gente = bark at everyone

This is a very natural way to express who the barking is directed toward.

In English, we use at. In Portuguese, para often works in this kind of context to show direction toward people.

Na is a contraction:

• em + a = na

So:

• na rua = in the street / on the street

Portuguese commonly contracts em with definite articles:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

So em a rua is not the normal form. The correct form is na rua.

In this sentence, the most natural reading is that the dog may start barking at all the people who are in the street.

So:

para toda a gente na rua is most naturally understood as at everyone in the street.

Context could sometimes allow slightly different interpretations, but this is the clearest and most likely one here.

The word order can be changed somewhat, but the original is very natural:

Sem a coleira, a cadela pode começar a latir para toda a gente na rua.

Putting Sem a coleira first highlights the condition: without the leash.

You could also say:

A cadela pode começar a latir para toda a gente na rua sem a coleira.

But that version is less neat and can sound a bit more awkward, because sem a coleira feels like an important setup for the whole sentence, so putting it at the front works better.

So the original order is good because it is clear and natural.