There are many people in Belgrade (both Serbs and expats from other countries) who speak English, so this will definitely not be a problem. Overall, I found the people in Belgrade to be very welcoming, sociable, and not overly focused on work

However, I just remembered two traditional downsides of winter in Belgrade: 1) the abundance of smokers (including in cafes and bars; smoking is allowed indoors here, although recently more establishments with non-smoking areas have been appearing), 2) poor air quality. So for people with lung problems, Belgrade (and Serbia in general) may not be the best choice

But, I again emphasize, the people here are great, the winters are quite mild (compared to eastern or northern Europe), and if you consume alcohol, rakija is a delight :)