The official language of Thailand is Thai. We speak Thai language in our daily life and there are four major regional dialects: Northern dialect, Northeastern dialect, Southern dialect, and Central dialect. For me, who was born in Bangkok, frankly I could communicate in only Central dialect, and could partially understand the other dialects.
As Bangkok is a capital city of Thailand and it is a tourist destination, most Thais living in Bangkok can speak and understand English. There are people that are fluent in both English and Thai. Their English skills are adequate for many purposes. However, since English is not our first language, many of us are not fluent in English, and do not speak it very well. Moreover, we are shy and nervous when speaking English, even we have learned English language for many years. So, please bear in mind that there are chances that you would meet Thais who cannot speak English at all.
In short, I would say that in Bangkok, you can expect to meet people who are able to communicate in English language with you. But, there are people that cannot speak English. So, if you plan to wander around Bangkok by yourself and you cannot speak Thai at all, I recommend you to install a dictionary application on your mobile phone. It would help you greatly.