Treba mi kupiti još dva balona prije nego što gosti dođu.

Literally, it is something like:

• treba = it is necessary / needed
• mi = to me / for me
• kupiti = to buy

So the whole start of the sentence is literally closer to It is necessary for me to buy...

In natural English, that becomes I need to buy...

This is a very common Croatian way to express necessity.

mi is the unstressed dative form of ja (I).

Here it marks the person affected by the necessity:

• treba mi = I need
• treba ti = you need
• treba nam = we need
• treba joj = she needs

So mi does not mean a direct object here. It is the person for whom something is necessary.

Yes. Many Croatian speakers say:

• Treba mi kupiti još dva balona
• Trebam kupiti još dva balona

Both are used to mean I need to buy two more balloons.

A traditional grammar approach often preferred the more impersonal construction with treba mi + infinitive, but trebam + infinitive is also very common in modern usage.

Because after treba / trebam, Croatian normally uses the infinitive:

• treba kupiti
• moram kupiti
• želim kupiti

So kupiti is the expected form here.

A structure with da + present is not the standard Croatian choice in this kind of sentence.

Here još means more / additional / another.

So:

• još dva balona = two more balloons

It does not mean still in this phrase. It is adding quantity.

Because balon is a masculine noun.

• dva is used with masculine and neuter nouns
• dvije is used with feminine nouns

So:

• dva balona = correct
• dvije knjige = correct, because knjiga is feminine

After the numbers 2, 3, and 4, Croatian uses a special counting form.

So you say:

• dva balona
• tri balona
• četiri balona

not:

• dva baloni

For many masculine nouns, this form looks like the genitive singular. For learners, the useful rule is simply: after 2, 3, 4, the noun usually does not stay in the normal plural nominative form.

prije nego što means before when it introduces a full clause:

• prije nego što gosti dođu = before the guests arrive

You may also hear prije nego gosti dođu, especially in less formal speech, but prije nego što is a very common and safe full form to use.

Croatian often uses the present tense in subordinate clauses that refer to the future, especially after time expressions like:

• kad
• dok
• prije nego što

English does something similar:

• before the guests arrive

  not

• before the guests will arrive

So gosti dođu is normal Croatian here.

Because dođu comes from doći, which is perfective.

That means it presents the arrival as a single completed event: the moment when they arrive.

• dođu = arrive
• dolaze = are coming / come repeatedly / are on the way

In this sentence, the meaning is before the guests arrive, so the perfective dođu is the natural choice.

Croatian often leaves out subject pronouns when they are not needed.

Here, mi already tells you who the necessity concerns, so ja is unnecessary.

You could add ja only for emphasis, for example if you want contrast:

• Ja trebam kupiti još dva balona, a ti hranu.

But in a neutral sentence, leaving it out is normal.

mi is a clitic: a short unstressed word that normally goes in the second position of the clause.

So:

• Treba mi kupiti... = natural
• Mi treba kupiti... = not the normal standard order

Croatian word order can be flexible for emphasis, but clitics like mi still usually stay near the beginning, in second position.