When a book is written for Packt (this is my experience):
1) The author submits his draft chapters to reviewers (there are two or three reviewers per book) - and they provide feedback. The author then has to address all comments from the reviewers, either to correct something (or to make it clearer and understandable) or to reply why it was not corrected. And this goes back and forth until everything is sorted out.
2) After that, technical editors review all chapters and if they feel that something may not be clear enough to the reader or it may have to be double-checked by the author, they will contact him and he'll have to update his drafts again.
3) Then, when the technical editors finish their thing and provide pre-final copies of the chapters, the author has to proofread everything, in order to make sure that everything is as it should be. If the author finds that something is not right he sends feedback to the editors.
You can see that there are many people involved on Packt's side, and (in my case) every one of them cares about quality. At the end of the book the author is even contacted to give an opinion on the quality of the technical editors he was working with.
I know this because I recently authored a book for Packt, named "OAuth 2.0 Identity and Access Management Patterns". It was released few weeks ago.
About the book, I posted an announcement on my blog: http://blog.thisismartin.com/book-oauth-2-dot-0-identity-and...
It is really sad if someone who doesn't know English well accepts an offer from Packt, as I can see in the comments here - there are some cases like that. I hope that they will improve in their selection of authors.