In the Editor, at the top of each file, there should be something like this:
Code:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="en" xml:lang="en">
Make sure it's not lang="en-gb" (<- fixed -- thanks @DrChiper).
Also, check the text, itself, for "lang=". There might be bits of text designated as being in a different language.