You don't like Asian people speaking to other people on their phones in their own language? It's none of your business, if they were speaking to you in their native language maybe you'd have a point, but they're not speaking to you so why are you so bothered?
As for your girlfriend, her family probably want to help you understand their culture. They're not expecting you to stop speaking English, they'll just want you to understand stuff they're saying so you don't feel like an outsider.
Just because somebody moves to Britain, does not mean they suddenly have to lose their mother tongue and start eating fish and chips all day :|