That REALLY depends on the parish.
At our church, half is in English, a quarter in Slavonic and a quarter in Serbian. Of course, that depends on the day. If we have a lot of English speakers, more will be in English. More Serbs, Serbian. More Russians, Bulgarians, Ukrainians, etc., more in Slavonic. Some churches do everything in English. Some do everything in Slavonic. If that is the case . . . good luck. I'm used to Slavonic and Serbian, although I am not a Slav, so I can go any which way the service goes. That just comes with time and exposure to the languages.
I've been to our church and been very happy with how open and friendly our church is. I've also been to our church and been utterly ashamed at the way some Serbs treat non-Serbs. It really depends on the focus of the parish (outreach to the community or outreach to a large population of Serbs). While trying not to make a judgement, some churches are ethnically focused and the spiritual life of that parish *may* suffer, especially regarding the outreach to the community as a whole.
I honestly cannot think of anything unique about a Serbian church from other Orthodox Churches that a visitor would need to know before visiting. Don't put your hands in your pockets. Learn to say "Vaistinu voskrese" as a response to Christos voskrese. You don't have to do so, but it goes a long way to show you are respectful to some Serbs who may not speak English well and who will appreciate a warm greeting of Christos voskrese, even if you can't say anything else.
There should be a service book that has one side in English and one side in Serbian/Slavonic. You should be able to track where you are and you will even start figuring out some Serbian and Slavonic words as you go on. Because of the season, it will be more difficult to follow in the book, as there will be special troparia, kontakia, antiphons, etc. Just flip around and you'll figure it out.
The choir may or may not be amazing. I hope the choir where you go is excellent.
They may or may not have a meal after Divine Liturgy. If they do not, hang around and talk to folks. If they do, go to the meal and talk. Speak with the priest. Enjoy yourself.