As others said: the rough language is inappropriate.
But asking who's paying is probably either a local culture of taking turns rather than making the wait staff deal with splitting every check, or "who's going to put this on expense account" or something else harmless. To find out which, ASK.
Personally, I would find the language made it an uncomfortable place to work, and would re-start my job search. You have the perfect answer to why you want to leave your current job: hostile working environment. Pity you weren't able to detect that during interviewing, but it sounds like that's their fault for not letting you see what the team was really like.
One thought before you do start planning to leave: ask your management if this is typical or if they are just hazing the new kid. Not that hazing makes it more acceptable, but that would suggest they'll become slightly less obnoxious in a week or two.
Or management may tell you that they're about to fire the loudmouths anyway, which might also mean the tone will improve somewhat.
Or this may in fact be how these folks normally speak. In which case you either find a way to tolerate it or leave.
So --again, my own reaction,not necessarily the best reaction -- I'd start planning an exit strategy. You may need one.