If Charlie's kids have lived in America long enough to learn to speak perfect English, then why hasn't Charlie himself?
The situation where a parent from the homeland struggles with English while the kids speak it perfectly and often translate for their parents is so common as to be a cliche (see any book by Amy Tan, who never forgets having to do this for her mother). Since Charlie is based on Det. Chang Apana of the Honolulu Police, we can make a few guesses. Apana was born in Hawaii to Cantonese parents (Biggers specified Charlie was too). Family then moved back to China, then he returned age 10. This can affect language acquisition, besides different people have different linguistic adaptability. Apana was fluent in Hawaiian, Cantonese and a couple other languages but not English and never learned to read or write, except in Hawaiian (Charlie was described as quite literate including in English). He had sons or nephews read him the daily newspapers and translate for him at movies. Obviously he had sufficient English for police work and had coworkers who could interpret. In Hawaii at that time, you could go your whole life not speaking English or knowing just enough to get by. Charlie's kids are supposed to have been encultured and educated as "thoroughly modern" (read: mainland) Americans who speak perfect English. In reality, they might have spoken English at school/work and Cantonese at home. But watch #1 Son Lee as portrayed by Keye Luke. He was raised in Seattle by Cantonese-speaking family and you can hear his slight accent and his fluent Cantonese in several of the Chan films.
