Interesting choice. I don't think I've heard of a stag do to Saint Petersburg before.

SPB has quite a range of bars, and at least in the centre you'll usually find someone who speaks some English in most bars. I don't think Russian bars are particularly 'friendly', but hey it's Russia! I don't think you'll have any trouble though.

Dumskaya Street is a very popular/ the most popular bar street, and it has maybe 20 bars on it. It's quite grotty and very hipster-ish, but is pretty interesting and is right in the centre. There are a few chain bars which tend to be much cleaner and less grotty, and they also have English menus. There are also a few nightclubs in the area.
Nearby is a cool bar called BeerGeek, which I really liked. You'll definitely need a map and some instructions to get there though. It's very well hidden.

Konyushenaya Square also has lots of bars and restaurants. It's nicer than Dumskaya, but I prefer the character of Dumskaya.