Croatia is the, next to Slovenia, the easiest country of the former Yugoslav states, to ride.
Pretty much everybody speaks English, there’s a lot of tourism (particular along the Adriatic Sea), and it’s an EU country.
It goes south along the Adriatic Sea, interrupted by Bosnia for some kilometers, to continue to Dubrovnik, which is also Croatian. It also stretches all the way to Serbia in the East. If you want a touch of Balkan, but stay in EU, you can swing down into Croatia, make you way up again, East, then just before Serbia, ride into Hungary for a short while, just to enter Romania after a 180 km or so.
Yes. Croatia is part of EU, but not Schengen.
Green insurance card: No
Technically not needed, being an EU member country. And they will not ask for it. But get it anyway.
Vehicle Registration Certificate: Yes
Yes, always. The original.
For select highways. Even for motorcycles. Toll is collected at physical gates on the highways.
Croatia’s roads a generally good. Even their gravel roads. At least those frequently used.
Road Safety: Good
Roads are good marked. The Adriatic Coast route can be a bit congested.
As with any tourist attractions, if you go to any, watch out for pickpockets.
Lots of motorcycle riders find their way to Croatia, so seeing a motorcycle is as common as anywhere else in Western Europe.
Cell phone coverage is good, except in the mountains. As they are EU, you should be home free on roaming if you’re with an EU based operator.
As it the rest of Europe: It is fine, but never as good as the one at home.
Most Balkan countries speak their own version of Serbo-Croatian, and Croatia is no different. Pretty much all speak English.
No need to learn Cyrillic.