Why do we hear "English" speech in Philadelphia in episode 3?
I believe the first reason is because it's a popular English language nursery rhyme. Translating it to Japanese (or other language) might remove the nuance.

The second reason which I noticed when watching this is because the conversation was heard from John's point of view, not Mari. Given that he didn't go to school because he's a slave, the conversation in English that he heard might not even be understood clearly by him. Thus the next 2 lines, "Good, Laura!" and "Thank you very much" were not translated too.