Što se mene tiče, kupujemo višednevnu ulaznicu u pretprodaji.

Yes, it is a very common fixed expression. It means as far as I’m concerned, as for me, or when it comes to me/my opinion.

Literally, it is something like what concerns me, but in normal English you would not translate it word for word.

Examples:

• Što se mene tiče, možemo krenuti. — As far as I’m concerned, we can leave.
• Što se mene tiče, to je dobra ideja. — As far as I’m concerned, that’s a good idea.

It is often used to introduce your opinion or decision.

Because after ticati se or in the fixed phrase što se ... tiče, Croatian uses the genitive.

So:

• ja = I
• mene = me / of me (genitive and also accusative form)

In this expression, you need:

• što se mene tiče — as far as I’m concerned
• što se tebe tiče — as far as you’re concerned
• što se njega tiče — as far as he’s concerned
• što se nas tiče — as far as we’re concerned

So even though English uses I or me differently, in Croatian this expression always takes that oblique form like mene, tebe, nas, etc.

Because Što se mene tiče is an introductory phrase. It sets the frame for the rest of the sentence, similar to English As far as I’m concerned, ...

So the sentence is naturally divided like this:

• Što se mene tiče, = as far as I’m concerned,
• kupujemo višednevnu ulaznicu u pretprodaji. = we’re buying a multi-day ticket in advance / in presale.

The comma helps show that the first part is a detached introductory comment.

In Croatian, the present tense is often used for a decision, plan, or something arranged, especially in everyday speech.

So kupujemo can mean:

• we are buying
• we’re going to buy
• we buy, depending on context

Here it sounds like a decision that has just been made or a plan that is now settled:

• Što se mene tiče, kupujemo... = As far as I’m concerned, we’re buying...

A more explicitly future version would be:

• Što se mene tiče, kupit ćemo višednevnu ulaznicu u pretprodaji.

Both are possible, but kupujemo sounds very natural in conversation when talking about a decided plan.

It is accusative singular feminine, because it is the direct object of kupujemo.

Base forms:

• višednevna = multi-day
• ulaznica = ticket / admission ticket

In the accusative singular feminine:

• višednevna → višednevnu
• ulaznica → ulaznicu

So:

• Kupujemo ulaznicu. — We’re buying a ticket.
• Kupujemo višednevnu ulaznicu. — We’re buying a multi-day ticket.

The adjective must agree with the noun in:

• gender
• number
• case

Višednevna means lasting several days or multi-day.

So višednevna ulaznica is a multi-day ticket, meaning one ticket that gives entry over several days, for example to a festival, fair, or event.

You can break it down like this:

• više = more
• dan = day
• višednevan / višednevna / višednevno = lasting several days

It is a common type of compound-style adjective in Croatian.

Yes, in many contexts you could say kartu, but there is a nuance.

• ulaznica = admission ticket, entry ticket
• karta = ticket, card, map, depending on context

For an event, festival, concert, cinema, or museum:

• ulaznica is often more precise, because it specifically emphasizes entry
• karta is also very common in everyday speech

So:

• kupujemo višednevnu ulaznicu = slightly more specific/formal
• kupujemo višednevnu kartu = also natural in many contexts

If this is about access to an event, ulaznica is an excellent choice.

U pretprodaji means in presale, on presale, or more naturally in English, in advance / during the presale period.

• pretprodaja = presale, advance sale
• u pretprodaji = in the presale phase / through presale

So the sentence means they are buying the ticket before regular sales or at the advance-sale stage.

For example:

• Ulaznice su u pretprodaji od ponedjeljka. — Tickets are on presale from Monday.
• Kupili smo ih u pretprodaji. — We bought them in advance / in presale.

Because the preposition u can take either:

• accusative for movement into something
• locative for being in something / within some state or setting

Here, u pretprodaji uses the locative because it means in the context/state of presale, not movement.

So:

• pretprodaja = nominative
• pretprodaji = locative singular

Compare:

• u kući — in the house
• u školi — at school / in school
• u pretprodaji — in presale / on presale

It is a standard prepositional phrase.

Yes. Croatian word order is quite flexible, although some versions sound more natural than others depending on emphasis.

The original:

• Što se mene tiče, kupujemo višednevnu ulaznicu u pretprodaji.

Possible variations:

• Kupujemo višednevnu ulaznicu u pretprodaji, što se mene tiče.
• Višednevnu ulaznicu kupujemo u pretprodaji, što se mene tiče.

But the original version is the most natural if you want to start with your opinion or stance:

• As far as I’m concerned, ...

Changing word order changes emphasis, not the core meaning.

Yes, sometimes it can.

Depending on tone and context, it can sound:

• neutral: as far as I’m concerned
• firm: for all I care / as far as I’m concerned, that’s settled
• slightly dismissive: as far as I’m concerned, do whatever you want

In this sentence, it sounds like a fairly firm opinion or decision:

• Što se mene tiče, kupujemo višednevnu ulaznicu u pretprodaji. This suggests: my vote is yes; I think that’s what we should do.

So the phrase is not rude by itself, but intonation matters a lot.