The official language of Bangkok is Thai and it is widely spoken in the country of Thailand. English is also spoken and many understand it but the official language is certainly Thai. English is more used in the cities and more modern places than in the countryside filled with natives of the land. It is broken when used but one can usually somewhat get the drift if they really want or need to. The point is that Thai is used because that is the country you are in and they are not too into using another language, except for tourist communication, when their heritage is of the Thai language.
You won't make a phone call and push one if you speak another langage like in the United States. If you go to Thailand, you'll want to at least know the language a little bit. There are some other languages spoken there but they are usually just spoken by the tourists who have come to visit from another land. Thailand people are proud of their country and their language too. They will accomodate English and other language only so far and that is simply when it is beneficial to them which is fine. They have the right to do so and that...they do.