According to the Croatian Bureau of Statistics, over 80% of Croatians speak English! You'll be hard-pressed to find someone who does NOT speak English to at least some degree. Under-30s in particular tend to have an excellent grasp of English. In fact, according to the 2019 English Proficiency Index test, Croatia ranks 14th in the world for proficiency among non-native speakers and it is the 2nd most fluent country in Eastern Europe.
In addition to being a mandatory subject in elementary and secondary schools, many Croatians grew up watching English movies with Croatian subtitles. Dubbing is a relatively new phenomenon here and video games are only just now being localised in Croatian. For years, the most popular children’s channel in the country was the US station Cartoon Network, so most young adults have encountered English frequently from an early age.
If you address older people in English, the results may vary. In general, people in big cities such as Zagreb, Split, Rijeka, and Osijek are significantly more likely to speak English than people from rural areas. Likewise, most people in tourist hotspots have at least a good working knowledge of the language. That said, if you do find yourself out in the country and stumble across someone who doesn’t speak English, body language can go a long way. Or just ring up a licensed interpreter from Rent-a-Local!