Brojčanik je naopako; poruku pročitaj unatrag dok traje odbrojavanje.

Naopako means upside down, back to front, or the wrong way round. It describes the state or orientation of something.

Unatrag means backwards or in reverse, especially in direction or order. So here, the brojčanik is naopako, but the poruka should be read unatrag.

Croatian does not have articles like English a/an and the. Whether something is definite or indefinite is usually understood from context.

So brojčanik can mean the dial and poruku can mean the message without any extra word.

Because poruku is the direct object of pročitaj. The basic form is poruka, but feminine nouns like this take -u in the accusative singular.

• nominative: poruka
• accusative: poruku

So pročitaj poruku means read the message.

Pročitaj is the 2nd person singular imperative of pročitati. It means read! when speaking to one person.

If you were speaking to more than one person, or using formal you, you would say pročitajte.

It is not stated, because Croatian often leaves subject pronouns out. The verb ending already shows who the command is addressed to.

So pročitaj already means you read! or read! to one person. You could add ti, but that would usually sound emphatic.

This is a question of aspect.

• pročitati / pročitaj = perfective, focusing on completing the action
• čitati / čitaj = imperfective, focusing on the process, repetition, or ongoing action

Here the instruction is a one-time action with a clear result, so pročitaj is the natural choice: read it through / read it completely.

Here dok means while. The verb traje is the present tense of trajati, meaning to last or to continue.

So literally, dok traje odbrojavanje means while the countdown lasts or more naturally while the countdown is going on.

Here it is a noun. It means countdown or counting down as a process.

In the clause dok traje odbrojavanje, odbrojavanje is the subject of traje. So the structure is basically while the countdown lasts.

Croatian word order is more flexible than English word order because case endings show grammatical roles. Both poruku pročitaj unatrag and pročitaj poruku unatrag are correct.

Putting poruku first gives it a bit more focus or emphasis. It can sound slightly more pointed or stylistically marked, which fits an instruction like this.