O personal trainer está no vestiário.

Because Brazilian Portuguese often borrows words and expressions from English, especially in fitness, business, and technology. Personal trainer is very common and natural in Brazil.

You may also see treinador pessoal, which is a more fully Portuguese version, but in everyday Brazilian speech, personal trainer is extremely common.

O means the.

It is the masculine singular definite article. Portuguese normally uses articles with nouns, even borrowed ones, so o personal trainer means the personal trainer.

If the trainer were female, many speakers would say a personal trainer.

Portuguese usually uses estar for location and temporary states.

So:

• Ele é personal trainer. = He is a personal trainer.
  
  • This is about identity or profession.
• Ele está no vestiário. = He is in the locker room.
  
  • This is about where he is.

That is why está is the natural choice here.

No is a contraction of:

• em = in, on, at
• o = the

So:

• em + o = no

That means no vestiário = in the locker room.

This kind of contraction is very common in Portuguese.

In this context, vestiário means locker room or changing room.

At a gym or sports facility, it is the place where people change clothes and may have lockers, benches, and showers.

It is not exactly the same as banheiro, which means bathroom/restroom.

Yes. The borrowed expression often stays the same, and the article shows the gender:

• o personal trainer = the male personal trainer
• a personal trainer = the female personal trainer

So Portuguese grammar still applies, even though the term comes from English.

Usually no, not in normal standard Portuguese.

A singular countable noun used as the subject normally needs an article or some other determiner. So:

• O personal trainer está no vestiário. = natural
• Personal trainer está no vestiário. = sounds incomplete or unnatural to most speakers

The accents mainly show stress.

• está: the stress is on tá
• vestiário: the stress is on á

So you say:

• es-TÁ
• ves-ti-Á-rio

In está, the accent also helps distinguish it from other similar-looking forms.

A simple approximation is:

ves-ti-AH-rio

with the strongest stress on AH.

In many Brazilian accents, the ti can sound a bit like chee, so you may hear something close to:

ves-chee-AH-ryoo

You do not need a perfect English-style approximation. The most important thing is to stress á correctly.