How would someone in Croatia, upon getting a criminal conviction for speech in Croatia, put a US citizen in prison or even collect on the judgement?
They wouldn't. US courts won't recognize or enforce Croatian orders as they don't live up to the same standard to have any reciprocal rights and agreements.
I know which country I'd be more concerned about having a court judgement in, and them being able to reach me. For starters, which country is more likely to be a travel destination that you'd then need to avoid?
I'm sure parts of Croatia are lovely and nowhere near as miserable looking as Lloyds new address, but there are nicer European countries to visit and not being able to visit the US would be far more inconvenient and impactful.
They have 3 F1 races a year for starters.