This is Serbian actually. I'll make some corrections so the learners would know what's correct.
1) Dolaziš danas, ali ja sam zaboravio da posle posla, kao što sam ti rekao, idem pravo na slavu. Koliko si dugo u Beogradu? Molim te reci mi da se vidimo.
You are coming today, but I forgot that after work, like I told you, I'm going straight to a Slava. How long are you in Belgrade? Please tell me so we can see each other.
2) Rekao sam da najverovatnije neću jer radim. Kako da ti kažem kada mi nisi ni rekao br
I told you I most probably won't, because I'm working. How do you want me to tell you, when you didn't even say...
I don't know what "br" means, maybe "broj telefona" (telephone number)
3) Mogao si barem da kažeš da neceš doći, jer sam te sinoć čekao. Tu sam do četvrtka.
You could have at least told me you weren't coming, because I was waiting for you last night. I'm here till Thursday.
4) Imaću sutra vremena od 15 časova, ne znam kada radiš.
I'll have time tomorrow from 3 pm, I don't know when you are working.
5) pa ako si za piće, večeru ili nesto treće, javi se
Well, if you feel like going out for a drink, dinner, or something else, give me a call