Poslije sastanka svratila sam u mesnicu po piletinu, ali je mesar rekao da je sve već prodano.

Croatian commonly forms the past tense with two parts:

• the l-participle: svratila
• the present-tense auxiliary to be: sam

So:

• svratila sam = I dropped by / I stopped by

This is the normal past-tense pattern in Croatian:

• radio sam = I worked
• došla sam = I came
• rekao je = he said

So svratila sam is not unusual at all; it is the standard past form.

The ending shows the gender of the speaker in the past tense.

• svratila sam = I dropped by, said by a female
• svratio sam = I dropped by, said by a male

In Croatian past-tense forms, the participle agrees with the subject in gender and number.

The same thing happens later in the sentence:

• mesar rekao = the butcher said
  because mesar is masculine

Svratiti means to drop by, to stop in briefly, or to make a quick stop somewhere.

So svratila sam u mesnicu is not just I went to the butcher’s shop. It suggests a short, practical visit, something like:

• I stopped by the butcher’s
• I popped into the butcher’s

That makes it a very natural verb in everyday Croatian.

Because poslije takes the genitive case.

• sastanak = meeting
• sastanka = of the meeting / after the meeting

So:

• poslije sastanka = after the meeting

Other common time expressions work similarly:

• poslije ručka = after lunch
• poslije škole = after school

You may also see nakon sastanka, which means the same thing and also takes the genitive.

Because Croatian uses different cases after u depending on whether you mean movement or location.

• u + accusative = movement into/to
• u + locative = being in

So:

• svratila sam u mesnicu = I stopped by / went into the butcher’s shop
• bila sam u mesnici = I was in the butcher’s shop

Here there is movement toward the shop, so accusative is used: mesnicu.

They are related, but not the same:

• mesnica = butcher’s shop
• mesar = butcher

So in the sentence:

• svratila sam u mesnicu = I stopped by the butcher’s shop
• mesar rekao = the butcher said

This is a useful pair to remember:

• pekara = bakery
• pekar = baker

In this sentence, po + accusative means to go get, to pick up, or to buy/fetch something.

So:

• svratila sam u mesnicu po piletinu = I stopped by the butcher’s to get some chicken

This use of po is very common in everyday speech:

• idem po kruh = I’m going to get bread
• otišao je po djecu = he went to pick up the children
• skoknula sam po mlijeko = I popped out for milk

Here piletinu is in the accusative because po in this meaning takes the accusative.

Here piletina means chicken meat.

That is the normal word when talking about chicken as food:

• kupiti piletinu = to buy chicken
• pečena piletina = roast chicken

If you want to refer to the animal itself, Croatian more often uses words like:

• pile = chick / young chicken
• pilić = chicken, especially a young one

So in a butcher’s-shop context, piletina clearly means the meat.

This is because je is a clitic. Croatian clitics usually go in the second position of the clause.

So after ali = but, it is very natural to say:

• ali je mesar rekao...

This word order is standard and very common.

You may sometimes see or hear slightly different word orders in real speech, but for learners, it is best to remember that short forms like sam, si, je, smo, ste, su tend to appear very early in the clause.

Prodano means sold. It is a participial form used like an adjective or in a passive-like construction.

So:

• sve je već prodano = everything has already been sold

The structure is:

• sve = everything
• je = is/has
• već = already
• prodano = sold

This is a very natural way to say that all the goods were gone.

You could think of it loosely as:

• everything was already sold
• everything had already been sold

Because it agrees with sve.

Grammatically, sve is treated as neuter singular, so the participle also appears in the neuter singular form:

• sve je prodano

Compare:

• piletina je prodana = the chicken was sold
  because piletina is feminine singular
• proizvodi su prodani = the products were sold
  because proizvodi is masculine plural
• meso je prodano = the meat was sold
  because meso is neuter singular

So prodano matches sve, not some hidden plural noun.

Već means already.

So:

• sve je prodano = everything is sold
• sve je već prodano = everything is already sold

It tells you that by the time the speaker arrived, it was too late because the chicken had already been sold.

Yes. Croatian has several natural ways to express this idea. For example:

• ...ali je mesar rekao da je sve već prodano.
• ...ali je mesar rekao da je sve već rasprodano.
• ...ali je mesar rekao da se sve već prodalo.

These are similar, but not identical in nuance:

• prodano = sold
• rasprodano = sold out
• se prodalo = everything got sold / sold out

In your sentence, prodano is perfectly correct and natural.