People don't tend to put so much thought and effort into one area and then completely slack and make mistakes in another. All that time and effort into a backstory and thinking through the details but to mess up on the broken english? People are very predictable, they would make consistent mistakes (or no mistakes at all). Not saying I am right, but rarely are people thorough in one area and completely lacking in another.
I think it is hard for someone literate to use broken English long term.