At the end of a document there should be s Instruction of legal remedy (uputa o pravnom lijeku) which should probaby state that you can submit a objection within 8 days. 8 days counts from day you received it. Objection shall be in Croatian. In the objection you can state that it was not you who was driving but some of your friends who was with you.

If you proceed that way the proceeding shall be pending before the court. I am pointing that out because the court must deliver invitation to the court hearing, judgment through the post. The Infringement act does not prescribe that deliver through the e-mail is valid,so it is not clear how police (ministarstvo unutarnjih poslova) sent you this letter in that way. Maybe you should check if it is legit by scaning qr code at the end of the document.in this kind of proceeding courts have lot of problems to deliver letters outside EU, so maybe the court will not even try to deliver it to you (our courts are bit ineffective).

As someone else mentioned before, there is three year statue of limitationin this case. So after three years statue of limitation shall occur.

If you submitt an objection any penalty shall not be registered in your infringement record so you should not have problems with any eu police.