ex-british and american occupation, respectively

uh, no idea about Thailand tho

OH! I wonder why that didn't cross my mind. Thank you!

Are you actually asking why people speak English all over the world?

Well, English imperialism in India is obvious, as is the saturation and monopolistic dominance of Hollywood (while the anti-trust acts of the 40's took away the right of studios to own theaters in America, they still have free sway to manipulate exhibition in other countries). People also speak Manglish in Malaysia and Singlish in Singapore, which are combinations of Malay and other languages, and similar slang mutations of language occur in India, the Philippines and other countries. English can easily be conjoined with foreign languages as well.

English is to Southeast Asia what Aramaic was to ancient Persia, a language of business and cultural exchange that must be learned if a nation's people want to contribute in global trade with the Chinese and United States. The Chinese are soon to have more fluent English speakers than all the native English nations of the world combined, by the way.