How do you know that was the motivation?

p.s. I am an immigrant from China and have been in the US for a long time. I can tell you that I'd speak English with people of same background at work, school, and in most other business or social interactions. This is mainly out of the respect to the official language and frankly is expected of me in formal setting. I'd speak Chinese after work, in personal situations or when everyone involved understands/prefers mandarin. On transpacific flights, I speak with FA mostly in English whether or not s/he is or look Chinese/Asian; I do speak with fellow passengers both in Chinese or English depending on their needs (or choice of language).