Languages spoken by Thai nationals in Bangkok are Thai and if you are lucky, some English. English is bad but generally OK enough in tourist areas (don’t expect quick conversations though). Outside of tourist areas, English is generally very poor or worse than very poor…
I don’t think that you will need it but there are also some “Chinese” Thai who speak the chinese language used here. It’s dialect is close to Hokken, somewhat similar to Kantonese but not close to Mandarin. Some Chinese Thai’s also speak Mandarin.
Dialects of Thai spoken in Bangkok come from all regions of Thailand.
Thai’s are generally bad at speaking English, but they really want to help and that can help a lot. You need to smile and be friendly first of all, then be patient and speak slowly. And most problems can be cleared
Try learning some Thai at the World Word Exchange here