Depends on where you end up. In bigger cities and more touristy areas you can get-by with English just fine, in smaller places it's gonna be a problem and you'll want to learn at-least basic Croatian.

The younger generation usually do speak some English even in smaller "non-touristy" places.

The Croatian language seems fairly easy to learn to me but I'm biased on this being a native speaker of a Slavic language so your mileage may vary.

If you do decide to visit, I would recommend the Istrian peninsula around Pula, a beautiful Roman-style city. You won't have any issues with English there and you're gonna see much less tourists than you would in the more "traditional" vacation places like Zadar.

You can get reasonably priced accommodation in the surrounding towns and there is a huge nature reserve with beaches (cape Kamenjak). They charge a fee for entry but IMO it's quite affordable. The point of entry is the Premantura town but I would recommend against getting accommodation there as it's gonna be more expensive.

Personally I'd recommend looking for a BnB in Pomer, a small town 2-ish kilometers from Premantura. If you're a more "posh" traveler, Pula has nice and more-less reasonably priced hotels.

Also, you will want to rent a car there, the public transport is OK but won't get you everywhere...