Language Barrier-only English?

You will have no significant issues. Go and enjoy.

Ditto.
I have been to Italy, Spain, and Croatia at least four times each--guess where I found English most widely spoken?

I didn't know this, but there is an international agreement on traffic signs, so no need to worry there. There can be minor differences, so I always ask my husband (the driver) to google it before a trip. I am sure he never has, but we've always been fine. We did not drive in Croatia, but the roads look excellent on highways and on islands, it is rural.

We'd like to get back and do a driving trip, but the ferries are very nice and convenient. Depending on your ultimate itinerary, you may only need a car for a portion, which can be nice if it works out that way.

Before trips I put in a fair amount of effort trying to learn some of the basics of the country's language, but when I put my newly learned skills to use, and likely butchering the language beyond recognition, the locals just start speaking in English to spare both of us a waste of time.

Maybe the appreciate me giving it the old college try. Or not.

I spent about a month in Croatia a couple of years ago and had no trouble with communication.

Part of the fun of trip planning for me is learning key words/phrases before visiting a new country. Well let me say that I was almost shocked (and honestly a little bit disappointed!) at how widely English was spoken and posted on the tourist path in Croatia. And it’s not just for Brits, North Americans, etc. All the other tourists (for example Germans, Italians…) are also using English in Croatia. You should expect very little language barrier. I encountered one single person that did not speak English, it was a bathroom attendant in Split.