L'agent parla amb el veterinari.

Because Catalan usually shortens the singular definite article before a vowel or mute h.

• el
  • agent → l'agent
• la
  • agent → l'agent

So the apostrophe shows that the vowel of the article has dropped. This is very common in Catalan.

It could mean either one.

Because both el and la become l' before a vowel, l'agent by itself does not tell you the gender. You usually need context, or another word in the sentence, to know.

For example:

• l'agent està cansat = the agent is tired (masculine)
• l'agent està cansada = the agent is tired (feminine)

Parla is the 3rd person singular present form of parlar (to speak / to talk).

Since l'agent is a singular subject, the verb must also be singular:

• jo parlo = I speak
• tu parles = you speak
• ell / ella / vostè parla = he / she / you speak
• nosaltres parlem = we speak
• vosaltres parleu = you all speak
• ells / elles / vostès parlen = they / you speak

So l'agent parla means the agent speaks / is speaking.

With parlar, Catalan commonly uses amb when you mean to speak with someone.

So:

• parlar amb algú = to speak with someone

This is a normal Catalan pattern. English may say talk with or talk to, but Catalan often uses amb in this kind of sentence.

Compare:

• parlar amb algú = speak with someone
• parlar de alguna cosa = speak about something

Because amb + el does not contract in standard Catalan.

Some common contractions are:

• a + el = al
• de + el = del

But:

• amb + el stays amb el

So amb el veterinari is the correct standard form.

Because veterinari is masculine singular here, so it takes the masculine singular definite article el.

• el veterinari = the male veterinarian / the veterinarian (masculine)
• la veterinària = the female veterinarian

The article also shows that we are talking about a specific veterinarian: the veterinarian, not just a veterinarian.

Yes, in this sentence it is very similar.

The basic order is:

• Subject: L'agent
• Verb: parla
• Prepositional phrase: amb el veterinari

So the structure is:

The agent + speaks + with the veterinarian

Catalan word order can be flexible, but this is the most neutral and natural order here.

The main stress is:

• a-GENT
• PAR-la
• ve-te-ri-NA-ri

Getting the stress right will already make your Catalan sound much more natural, even if your accent is still developing.