I'll def be going to Dubrovnik because it looks absolutely beautiful and as a fan of Game of Thrones I think it would be awesome to see where some of the scenes take place in person. I'll be going to some other European countries as well but it is my first time going to Croatia. What is there to do in Zagreb? And where are some other cool cities or towns I should check out? How about the food?
I'm from Croatia so I can give you a few advices.
Zagreb (my hometown) is a capitol of Croatia. The old town is cool, that is a part around center of the city. I recommend you to visit a Zagreb cathedral and the whole upper town ( Gornji grad ) in general. Lots of good restaurants with great food and great beer. There are also a few great museums in tthe center of the town.
Now, Plitvice lakes ( Plitvička jezera ) are a must visit location. it is the most beautiful and the best known national park in Croatia.
Plitvice lakes are some 150 km from Zagreb, you can be there in 1.5-2 hours.
When it come to the coast, I can recommend you Dubrovnik, Šibenik and Zadar. Beautiful towns, especialy the historic parts. Dubrovnik is well known worldwide but trust me, Šibenik and Zadar are also awesome. Also, beautifull sea, great restaurants with great food and wines.
The center and the historic part of Zadar:
City of Zadar:
Šibenik:
Historic part of Šibenik:
If you are interested in national parks and such things, definitely visit national parks Krka and Kornati.Both of those are near to Zadar and there are lots of organized trips to those places.
When it comes to language, you don't have to worry about that. Tourism is the main source of economy in Croatia and basicaly every person under 30 is fluent in english.
All in all, I hope this will be helpful to you. If you have any questions, feel free to send me a PM.