If Mulan's family speaks English as their first language in the live action movie, why do they have accents that suggest otherwise?

Unless chinese is their first language and all of china decided to speak English instead, it is an example of using english when we all know they wouldn't be way back before countries were friendly with each other and shared stuff about their languages and cultures.

It's like in doctor who, where the tardis translates everything for you with special science, but everyone will have accents relating to Earth. Accents that native English speaking would have (mainly accents originating in the UK).

It's not like they make up an accent an alien would have if they learned English. Secondly, when they go to countries that don't have many English speakers, their accents don't change at all! 

In Mulan, many characters have Chinese accents. But why not just have them speak Chinese or have an accent that native English speakers have? It just doesn't make sense to me.

Same with Uncle Iroh from Avatar: the last airbender. Yes, he is my favorite character from the show forever and always, but his father didn't speak another language, his brother didn't, and he doesn't seem to... So, why does he speak like it is his second language? 

So many questions...